I use a few avenues to find old games.
With Ebay, I first off tend to avoid buying first party stuff due to the inflated prices until I find it as part of a larger lot. So basically I wait until I find a decent lot with at least a few games I'm looking for and buy it that way. When you divide out the cost per game, it usually ends up being cheaper per game than buying single games at a time. Then, I trade the carts I don't need with other guys either here on CAG or on CL or whatever. I try to keep it 1 for 1 in most cases so I always have a flow of games going in and out. The stuff I really want for my personal collection gets "vaulted" and I keep it.
Another path is just bartering on Craigslist. Retro games unfortunately are commanding pretty high prices now compared to 10 years ago, so you have to get in where you can if low on cash. I've bartered other household shit from time to time for games and accessories. For example, I got a couple boxes of .270 by mistake and I don't have that caliber rifle, mine is a 30-06, and since ammunition is non-refundable I found some dude who was more than happy to give me a bunch of "crappy old" NES games for the ammo. I essentially paid 30 bucks of ammo for like 12 decent NES carts. Sometimes you get lucky.