Depends on your definition. When I was a kid, beginning to read comics, the mainstream included Archie, Harvey, Western (Gold Key), Dell (split from Western), Charlton, DC and Marvel. The non-mainstream, by that definition would be Warren and the Undergrounds. By that, it would have been when I flipped through an issue of Creepy, while I was in the hospital (my roommate had that, while I had a Teen Titans issue and we swapped), in the mid to late 70s.

I read Gold Key, Harvey, Archie, and Charlton from around the same time as Dc and Marvel. Gold key before, since Uncle Scrooge and Super Goof were my earliest comics. By the mid-70s, I had read an Atlas/Seaboard (Phoenix #4, the revamp issue), so I was aware that there was another publisher in town. Well, for about a minute, before they closed up.

For what I would really call the Independents, it was 1983, when I went into a local bookstore and discovered they had a comic rack again (they went back and forth with it) and they had a few things from Pacific, the earliest First comics, some Red Circle, Marvel's direct market attempts to flood the stands (the Baxter reprint comics) and the Nexus magazine issues. That was a true revelation of different material, when I was mostly done with DC and Marvel (except for X-Men, New Teen Titans, and Legion of Superheroes and a few issues of Daredevil). I picked up Jon Sable, American Flagg, Mighty Crusaders, Starslayer and flipped through Nexus. The next summer, I was off to college, where I entered my first comic shop, complete with Fanzines, British comics, Eclipse and more. The Union bookstore had Tintin and Asterix, plus Lone Wolf and Cub and some of the First Graphic Novels. I bought the first Calvin & Hobbes collection there. Another bookstore, The Little Professor Bookshop, had RAW magazine and Love & Rockets. I mostly just flipped through those, as I was still mostly in an adventure mindset, until I left college and started trying other things, leading to The Tick, Dark Horse Presents, Grendel, Maze Agency, and Lethargic Lad.

The real watershed was a shop in Charleston, SC, where I was stationed, called The Green Dragon. It was owned and operated by a couple of hippies who threw in all of their interests: comics, graphic novels, role playing game sourcebooks and accessories, New Age books, sci-fi books, pewter fantasy sculptures, incense, and martial arts equipment. Where else could you pick up an issue of Cerebus, a dragon statue, a book about mythology, a volume of the Wild Cards anthology series, a Prince Valiant collection, Pepe Moreno's Zeppelin and a pair of nunchucks? It was there that I picked up the first Cerebus phone book, the Fantagraphics Prince Valiant books, NBM's reprint of Corto Maltese, Catalan Communication translations of European comics, Titan Modesty Blaise reprints, Will Eisner's graphic novels (including a signed & numbered To The Heart of the Storm, and the limited edition hardcover (with slipcase) Monster Society of Evil reprint book ($75, but worth it!!!) From that point on, I was reading more indies (especially when Hellboy and the like came along) than DC or Marvel. I also discovered the Bud Plant Comic Art catalog, at that time, which brought me all kids of great stuff.

I started on comics in 1989, which was an odd time for Indy books. The immense wave of 1980s Indy publishers was largely on its way out by this point, and Image and Valiant were really going to screw with our understanding of (and appreciation for) Indy publishers a few years down the line.

I distinctly recall a massive batch of licensed property books by NOW sitting in the discount bin at my LCS, which pretty much meant I was never going to give them the time of day. The ones I remember most vividly were Speed Racer, which couldn't have seemed less interesting to me.



Beyond that, I wasn't really aware of anything beyond DC, Marvel, and Archie at first.

Then I got swept up in the Teenage Mutant Ninja Turtles craze. I bought some issues of the Mirage TMNT, though I'm not sure I consider that a true Indy publisher as they only ever had the one successful title, but, as part of that TMNT craze, one LCS I visited while seeing my grandmother was displaying all sorts of b/w Anthropomorphic Indy titles, and I grabbed my first issues of Usagi Yojimbo (published by Fantagraph at the time) because of this.



My first true Indy comic (by my own definition). Though I didn't really come to appreciate and love it until decades later, when I re-read it as an adult.

Then Valiant came along, and I saw the true potential of an Independent Publisher, not beholden to maintaining a status quo:



My first Valiant comic, which I admittedly picked up only because Wizard magazine told me to.

Granted, this wasn't a place where creators had carte blanc freedom like I might have seen at Pacific, Comico, or Eclipse a decade earlier, but it was still an alternative to the mainstream that I savored.

To this day, I'm not as well versed in Indy classics as I'd like to be, but I understand and respect their potential, and many of them are on my to-read list.

In fact, Grimjack is next on my To-Read list, my interest in it stemming out of Ostrander's work on Magnus Robot Fighter and Rai for the aforementioned Valiant Comics.

Good question. I really don't recall my first non-mainstream or non-newsstand comic. It would have been in about 1973. I was going to head shops to buy underground comix two years before I started smoking pot.

You didn't miss anything with Now, especially the Speed Racer comics.  They hired amateurs at cheap rates and the art reflected it.  Back when the Speed Racer live-action movie came out, they reprinted the material in trades, which ended up remaindered and dumped in our bargain section at B&N.  We couldn't give that junk away.  They tried to make it look like the Tatsunoko cartoon; but, the artists were no Ippei Kuri.

Now did have some better material,like when they started up the Green Hornet books.  They also hired a young painter to do a Terminator comic.



Some long-haired kid named Alex.....something...............

Back then, my LCS was about a half hour away from home. It was mainly a used bookstore owned and operated by a lovely woman named Jean. To supplement her sales, she carried mainstream comics and some indy publishers (along with copious boxes and bins of used/old/back issues).

Since it was so far and I had no driver's license yet, I had to wait for Mom to go to Durham and thus could stop by the mall where Jean's bookstore was located.

On one trip, I found and bought this:



I was looking for and buying up anything I could find related to Robotech. IIRC, in the show's end credits, there's an actual statement of 'Comic adaptions available through Comico'.

I'd never heard of Comico. Lo and behold, Jean carried them. From Robotech, I expanded throughout the rest of their quite remarkable line.

Though I was aware of several indy publishers, because of my limited ability to travel to Jean's bookstore, I wasn't able to follow any of them consistently. So until I did acquire my driver's license, I bought what I could when I could.

Atlas Comics. Although that was on the news stand. I guess...

First Comics. American Flagg! Nexus. Jon Sable.

DC & Marvel always were the majority of my books but thru the years I read stuff from Archie, Harvey, Dell/Gold Key, Charlton, Atlas, First, Comico, Eclipse, Valiant, Malibu/Ultraverse, Image, Dark Horse...