Edit: This is one of the most popular topics in my old blog. These links will certainly help you find that GBA rom you are looking for.
Are you still looking for that Grand Theft Auto, Yu-gi-oh, Final Fantasy, The Sims, Lord Of The Rings, or that elusive Kingdom Hearts rom? Here's some site that you might want to check out for GBA roms:
Roms Web-log -- It's a blog about GBA roms. They have downloads and links to other sites with GBA roms.
White Wings -- A GBA rom site with the latest GBA games. You need to register to d/l. (I fixed the link, it's now working fine)
Huge list of GBA rom resources -- A lot of links here.
LastEnd -- Another GBA rom download site.
Games2Play -- This site closed without warning, it may just be down for the moment or closed forever. (Still no word from this site)
That's all folks.
Edit: I have to add Romulation. This site needs registration, but it has every ROM for a lot of consoles, including Nintendo DS ROMS, PSX ROMS, maybe even Saturn ROMS.