All of that information is of course necessary, but not so much if you don't know where to look, and a few have been touched on already. Pawnshops, flea markets, garage sales, and donation stores (such as Goodwill) all have varying degrees of games. The key is usually luck. However, timing is also important. You're not the only one looking for these games, and the rush is fast. If a garage sale that listed video games in the paper starts at 9 AM, be there at 8:50. Same goes for flea markets. The bigger the city you live in, the more competition you'll have. That means even more work.
One of the keys is patience. If you're looking for a specific rare title, don't become frustrated after not finding anything at a few sales. It may take years before you stumble onto a legendary find. Don't just head to online auction sites. You'll (almost) always pay a large premium. Never count out any sale either, regardless of whether or not they have games listed in their ad. It's impossible to list everything the average person will have in front of their house. It's always worth a short walk or a drive.
Also, try to find one store and stick with it. Know the owners and chat with them regularly. Tell them you're interested, and in the case of pawnshops, most people won't turn down an opportunity to make a quick buck. They won't know people are interested if you don't tell them. If they're receptive to you, ask if they'll keep a list of games you're specifically interested in. Give them your phone number so they can contact you when that gem finally arrives.
If you have chosen to be a member of this quickly growing segment of the gaming industry, welcome. The most important thing to remember here is to have fun. Many people are caught up in the financial part of it all, when really you're doing this to play games. If you're not doing this for that reason, then this is not your hobby.