If you had to find/download just one Japanese artist, make it Yoko Kanno (& the Seatbelts). You may remember her from such soundtracks as Stand Alone Complex and Cowboy Bebop.

I've got a couple of her albums (notably the two above), and they are absolutely outstanding.

Other than Yoko, I recommend Mad Capsule Markets, Rip Slyme, Plus-Tech Squeezebox, and Puffy Amiyumi if you can find any. Most of the other stuff I've got/recommend is miscelaneous anime soundtracks - they just seem to have better music than Western cartoons.