This might be a crazy question here...

What is the best place to *gasp* BUY the old Pokemon games. I got into the game last year with Diamond and now with Soul Silver, but I'm interested enough to go back and play the old school games too. I already have a GBA SP, so all I need is a place to buy the games. I know Gamestop jacks up their prices - where do you suggest I go? Is there a place online or should I start hitting up garage sales?

Craigslist.org ?
i'm sure people sell old pokemon games

Ebay.I saw some for 5USD.

Do you mean the originals for GB or the GBA remakes?
i'd just get the remakes, unless you're collecting them or something.

You should be careful about buying them on eBay (or other sites that have sellers that aren't verified retailers); there are a lot of fake cartridges circulating there.
You could try asking your friends/family if they have any old Poke'mon games that they are willing to give/sell you.
I don't know what the odds are of finding them at a garage sale are. I think your best bet (unless you find cheap, legitimate copies online) is to look in some game shops that sell second-hand games.A word of warning though; the first and second generation games have batteries fitted into the cartridge that may not function properly now (I tried playing my old Zelda: Oracle of Seasons game on my SP a little while ago; it wouldn't save properly : ( ).
Good luck :)