(The term you're looking for is flashcard or flashcart.)
Do
not get a Supercard. It was designed first and foremost as a DS flashcart, and that came at the expense of proper compatibility with GBA games. They'll (usually) run, but they'll run poorly. (For DS games, it's also completely outclassed by modern flashcards - it won't work on anything newer than a DS Lite, and antipiracy measures will prevent many DS games from working. If you want a DS flashcard, get an R4.)
Anyway, Spruced is right; if you want a GBA flashcart, there are three options worth considering:
- The Everdrive GBA X5. This is considered the best GBA flashcart currently available. It loads games faster, it uses less electricity than other flashcarts (so you'll get better battery life), and it has built-in real time clock support. However, it's expensive (100 USD) and larger than a regular GBA cartridge - it sticks out a bit.
- The EZ-FLASH IV was for a long time the only GBA flashcart worth considering; all the other good ones had been discontinued. It's the same size as a regular GBA cartridge and much cheaper than the Everdrive, but it lacks RTC support. You can patch games like Pokemon so that the in-game clock will move forward while you're playing, but time will be frozen while you're playing a different game or the GBA is turned off. (The EZ-FLASH IV used to also work as a DS flashcart, but recent software updates removed that functionality because being a slot-2 flashcart made it outdated in ways that no amount of software updates could fix. Instead, the EZ-FLASH team focuses on making it the best GBA flashcart it can be.)
- The EZ-FLASH REFORM was recently announced. It's a modified version of the EZ-FLASH IV that comes with two cases - one GBA game sized case, and one DS Lite Slot 2 sized case. If you want to play on a GBA or an original DS, put the REFORM in the GBA one-sided case. If you want to play on a DS Lite, you can put it in the Slot 2-sized case and it won't stick out like normal GBA games would. The EZ-FLASH REFORM hasn't actually been released yet; if you want a GBA flashcart right now, your (good) options are basically limited to the IV and the X5.