Another option is the EZ Flash 3-in-1, but you need a DS/DS Lite and a DS flashcard for it to work. You can only load one game on it at a time, but it works like a real game, so you could use games that link up that way, such as with Pokemon migration. It also allows you to use it as a rumble pack or a RAM expansion for DS games, although the rumble feature needs patched ROMs, usually. I don't remember the exact details because it's been a while since I tried it.
The EZ Flash 3-in-1 is a very different type of GBA flash cart, but if that is the sort of thing you're after, that's the best option. If you want a traditional flash cart, then... well, everyone else has already talked subject to death, so I don't really need to repeat everything, do I?