You can try looking on
Play Asia. There are probably other sites that sell Japanese games but that's the biggest one I'm familiar with. A lot of people buy such things on eBay and Yahoo Auctions Japan.
There's also the option of buying a copier and playing the ROM on a real system.
All of Nintendo's handhelds are region-free, as far as I know. The original Gameboy definitely is and I'm sure the SP is too, seeing as how the GBC and GBA both were as well.