As others have said start in the marketplace and craigslist can be good source (use craigslist only after you get a feel for pricing) and the larger flea markets can sometimes hold treasures.
Like for example you may see a craigslist ad for a RARE VINTAGE KLASSIK ATARI!!!! and the pic if there is one will be a 4 switch covered in burnt moldy dirt with weeds growing out of the cart slot with several AWESOME!! carts like Pac Man,E.T.,Frogger(missing the label),Combat and Pitfall all for the low price of $200!!!! You probably want to avoid that deal
Welcome to Atari.
Weeds growing outta the cart slot is maybe the funniest, best image I've read this summer (and I read a lot)...well played, sir, well played