Another Famicom to GBA SP adaptor is on the market this time called the "GBA SP Time Machine". For the low price of $39 you can own one. No idea if it is any good but I suspect it is not. Still if you have a lot of Famicom carts, a GBA SP and need a way to to play them on the go, the price is right.
According to the vendor...
"GBA SP Time Machine
1. Play your 8bits Japanese Famicom carts on your GBA SP
2. Built-in speaker, earphone jack
3. GBA game cart and Japanese Famicom carts can plug into it simultaneously
4. Compatible with Japanese Famicom carts only
5. GBA SP console and cartridge are not included
Now you can play your Japanese Famicom carts directly on GBA SP without any emulator. A Time Machine which allows you to play 8 bits Japanese Famicom carts on your GBA has been developed!!! The Time Machine looks like a TV Tuner adaptor but, of course it is smaller than the TV Tuner. Japanese Famicom carts are plugged in at the back of the unit, while the Time Machine sits underneath the GBA SP. 4 AA size batteries are needed for this unit. It only compatible with GBA SP and Japanese NES cartridges. It is easy to use. Just plug it into the GBA SP and you will have fun of 8bits old games. It could be a great product for the Famicom fans who still have their Japanese Famicom carts.
GBA SP console and cartridge are not included"
Get yours at the link bellow.