For maintenance parts like oil filters and belts you can usually find a local equivalent.
For performance parts I've ordered from both blackhawkjapan and rhdjapan in the past, they are good 👍
For OEM I've ordered some stuff from Yahoo auction using jauce or even with Amazon Japan.
Tires though, that's a pita. I couldn't find anything in OEM size (165/55r15) except from a special order of nankang branded tires. I went with 195/45r15 instead. They are tight but they fit. I'd check local to you if someone can order tires from overseas, that could help a lot.