I've had great experience with GBA stuff from the UK-based store www.gbax.com. The owner is also the author of GP32 Doom, which shows he's interested.
My GP32 is coming from a private seller in the UK, but I would trust Craig from GBAX any time.
Nice summary. Let me correct a little problem I see in your writeup -- Wave started to correct this but I can add more detail.
The flash carts for the Nintendo Game Boy Advance are measured in megaBITS. 8 megabits equals one megaBYTE, which is a more common and useful measurement. So the...