The spendy adaptor is also almost impossible to find. It's called a.... well, I forgot, b/c I never had one. Basically, if you want to play Japanese Hucards that bad, just buy a Japanese PC-Engine or Core Grafx. They're very cheap, and not that heavy. I'm sure you could get one around $50 shipped if you're patient and know where to look. There are other adaptors but they're apparently all terrible. Again, never used one, but I was pretty active on a couple of PC-Engine forums for a a few years, and that fact was voiced over more than a few hundred times.
My bad, it's the PC Engine that needs to be soldered along with the card adaptor. I always get the two mixed up.
If I can find a cheap TG16 im modding it up to play Japanese cards. Chances are your right though I should just look for a PC Engine. I just love to mess with my hardware, I cant help myself.
Here's one: http://joyce.eng.yale.edu/~bt/turbo/files/country.switch and if that doesn't work: http://web.archive.org/web/20041030215933/http://joyce.eng.yale.edu/~bt/turbo/files/country.switch and as a bonus: http://web.archive.org/web/20041030215933/http://joyce.eng.yale.edu/~bt/turbo/files/pce.hardware.mod