The best way to develop GBA roms is using an emulator, both for the greater turn-around time, and the availability of easy debugging.
No one ever really produced "blank carts" for the GBA -- not as in "I can load my program on here and sell them". There were flash-carts, which are developer tools for people like yourself to be able to test, carry, and show off your work (and which could also be abused for piracy) though. Today, no one I'm aware of is still manufacturing or retailing any GBA flash-carts. The best you can do is probably Ebay -- see what turns up when you search for "GBA flashcart" and do your research on what turns up because plenty of flashcarts have limitations and bugs that you might be happy with. If you can find and EZFlash 4, though, that one seems to have the best reputation -- I picked one up on Ebay a couple months back for like $70.