I've really been wanting to get into GBA programming. I really would like to buy a cart to be able to develop directly on my DS ( I thought I would be able to find one in Akiba, but they seem a bit impossible to find in person, and all the online stores I've seen seem mega shady)

Practically speaking, most of your iteration will be on emulators anyway, so don't let not having a cart stop you from getting started. GBA programming is a ton of fun!

Here's a nice tutorial to get you started: http://dev-scene.com/NDS/Tutorials_Day_1