Okay thank you very much, I'm very sorry for posting in the wrong place I promise it was not on purpose
No worries, that is what the mods do. Move stuff from here to there in order to confuse people. :-)
As Ravyne mentioned, emulators are fast and easy and generally the preferred route for main development.
where I could get blank cartridges for the gba? Not that I'll be needing them for a while
Carts are painfully slow, and you need actual hardware. There are a few types of devices. There are the actual real flash cards that require special boxes to program. Alternatively, shop around and find adapters for SD cards, I remember the name SuperCard thrown around for GBA. Either way, you've still the the manual effort of copying the image to the card, ejecting the card, moving it to the device, waiting for the device, and the occasional data error.
It is far faster and more convenient to have the build tools that automatically reset the emulator when the image is built successfully.