Roaming on a 9000 in Japan works just fine. Docomo's coverage is TRULY exceptional throughout the country even on remote locations up the mountains or below sea level. Sadly data coverage is patchy at places with docomo. Personally i prefer camping with Softbank.
I'm on a world roaming plan so i don't really know where to get sim cards. if you're interested in prepaid plans there are a few kiosks in Narita Airport that sell prepaid phones or short term plans. KDDI Group, docomo, softbank, mobel, temco, digitel all have presence in the terminal to name a few. A lot of offices that rent an already activated used handset with a plan ready to go can also be found there.
I believe Softbank offers 3G simcards but i'm not aware if BB plans are available for them. They usually only need to have a look of your passport and the municipality registration card (gaikokujin toroku) usually helps for people that don't hold other forms of identification in the country.
CDMA is of course out of the question but an unlocked 3G GSM handset should work just fine. I've even roamed in the past with a 8703 with no problems at all with BB service.