Where can I buy a used motorcycle?

I would like to make a three-month trip around Japan by motorcycle.
Now my first question is where can I buy one? I only find export websites which ship bikes from Japan.
My second question would be, do I need other documents than a translated license? Obviosly I need insurance, but are there insurance companies in Japan which even insure for that short of a time?
I think that's it.
I would really appreciate it if someone has good answers to my stupid question. Any information helps.
Thank you very much.

probably you can't register it, because you are not a resident.

This is probably a better choice for short term.
https://www.rental819.com/english/

Sorry, I didn't see the '3 months period' lol

No resident people cannot motorcyle's resgistration.
(just buying possible but...)
If you have over three month visa you may get a "midium-long" resident card.
http://www.soumu.go.jp/main_sosiki/jichi_gyousei/c-gyousei/zairyu/engl...
But not sure all kinds of longer stay visa acceptable or not.
http://www.immi-moj.go.jp/english/info/index.html

Ask again at this forum.
http://www.gaijinriders.com/forum.php

Basically motorcyle rent service is not so good as rent fee if short.
(depends on plan and motorcyle class "cc = engine displacement")
(sometime will be campain=off price mainly off =out of peak season or unpopular ones)
Just motorcyles conditons good also supports.
(depends on shop, above shop "819" is very famous and service branches nation wide)