I've checked almost everywhere; gamestop, thrift stores, used game stores...any tips or places to find firered? leafgreen?
You can find loads on places like amozan Just type in something like (for example you want Pokemon ruby) Pokemon ruby then loads of results will come up but unfortunately nearly all of them if not all of them are second hand But you can get never opened ones They are rather cheep and if bought from a good seller will almost certainly work I'm not sure they sell new gba games any more so second hand is your best buy eBay is also a good site just make sure you check the sellers reputation
try online gamestops and get your credit cards set also try yard sales because they usually have at least 1 GBa game but if you have a dsi dont look for them.