EZ-Flash IV is the best GBA flashcart.
This shop in spain still has it in stock (it seems) and it´s rather cheap too:
http://dsflash.es/gameboy-advance/231-ezflash-iv-minisd.html
Listed as official reseller at http://www.ezflash.cn/dealers/.
When you get it, install this skin and you are prefect to go.
http://ezflash.sosuke.com/viewtopic.php?f=13&p=75502
I have a quick question for those who would know: would it be better to purchase the 3in1 or a IV if I am only interested in making, for example, a Mother 3-only cartridge?
I have a quick question for those who would know: would it be better to purchase the 3in1 or the IV for creating a custom cartridge only being used for Mother 3?
Oops, sorry. There doesn't seem to be a way to delete that.
As I understand it, you can modify a junk GBA cartridge and insert a 3in1, and I don't mind modifying a cartridge. Should I just do that considering it's cheaper than a IV?
Which would eliminate the flashcard's boot screen, right? Basically changing the flashcard into a single-game cartridge.
I'd like to create a custom cartridge with a label, box, etc. for a few Japanese-only games. I don't know, it'd feel more "authentic."
Sure it is, there are custom stickers you can print.