So, a student from my comic class asked me, "What do you think is the best and cheapest source to buy old Marvel comics from?"
Which sparked in me a huge Proustian Rush, responding:
"Boy. Tough question. Long answer. Problem is, I've never been a back issue guy. The only time I ever indulged in getting a complete series was with the Marvel "Star Wars" comic, and only then due to a confluence of factors:
1) Issue #48 was the comic that started me as a bona fide comic COLLECTOR (for better and worse), and so it held a real iconic power for me to 'complete' it,
2) This was waaaaay back before the days of trade paperbacks, and Marvel's "Star Wars" was highly serialized, with plot arcs running up to six issues and subplots running far longer - both of which were pretty unique at the time. As a genuine fan of longer form comics, I just liked them as comics. The fact that it filled my major Star Wars fix was also a plus.
3) I had several friends - Jim Doller primary among them - for whom comics were about the acquisition and hoarding of them as objects. Jim to this day carries around with him a list of 'needs' that he brings to conventions and stores. It was under their influence (I'm highly impressionable) that I thought somehow owning all the issues of the comic would fulfill me. It was only after I completed the run and spent a lot of money that I realized that that's not how I enjoyed comics. So I guess a valuable lesson was learned.
4) All that aside, the Marvel "Star Wars" had a really stellar roster of artists and writers - and what was interesting was that many of them were much older pros for whom SW meant nothing more than a regular monthly paycheck. Curiously, this made the comics much, much better than the efforts of younger artists and writers who toil away on Dark Horse's dreary and joyless SW comics. The Marvel SW was more like a fun, rollicking 1940's pulp style adventure strip, which even though had little to do with the films, had so much in common with the materials that inspired them that they captured the feel of the films far, far better. Another valuable lesson, there, somewhere.
Anyway, conventions are the place where dealers bring out the longboxes to sell, and usually will have dollar or fifty cent bins full of all sorts of goodies. If you're looking for something specific, some will have several thousand issues with them. Don't let yourself get cheated. If there's something you really want, buy a price guide, and comparison shop. (This is why conventions are the best for back issues.) Ask yourself if you're buying for the act of owning, or buying for the enjoyment of reading. Once you find a dealer with a good backstock collection, prices that don't seem insultingly usurious, and is nearby enough, make a point of paying them a visit when you want back issues. Dealers themselves are part of a network and if they don't have the back issue your looking for, them may be able to find someone who does.
There are also many online and catalog places to buy comics by mail. Mile HIgh Comics in Denver has long been the place for this kind of thing. I don't know how I feel about it, but it is an option.
What are you looking for specifically? Bear in mind that I have several thousand comics from the late 70's to today, with the bulk of them being Marvel & DC from 1981-1988. I'm willing to part with ones I have little attachment to for a very, very small fee, and outright donate some of those that I have lost interest in. However, I gather your tastes run to the Silver Age, which means that (a) I can't help you there, and (b) you'll have to have more money to spend with dealers, as those are regarded as more valuable.
To which she replied:
"Star Wars comics: there is very little geekier, or more fun.
Actually, I find sometimes that owning the whole series can be fullfilling. I have all but about four or five issues of the (2nd) Star Trek D.C. run, and am very proud. Star Trek comics obviously aren't rare or expensive in the least, but it's fun going back and reading them.
Q: Am I buying for the act of owning, or enjoyment and reading?
A: Both. It'd be cool to say "Oh, and I own an original copy of the Galactus Trilogy!" to people, for example, but I would really just have it because it's a great read, and it's cool to know you're holding the one that was the first.
You gathered right, of course, Silver Age Marvel comics, specifically Fantastic Four. I've read many different comics from that era, but the FF are my favorites. I am aware of how much they cost (I googled the particular issues extensively), and would like to save up for one...I have this habit of hoarding money."
So it looks like I'm already too late to save this poor child from the horrible, degrading life of a comic book collector. I swear, there must be some nicotine derivative in the magenta ink, or something...