there is a graff shop in upper Shibuya, i don't remember the name... but they sell all the euro paints and have a full selection of caps... its in this little music district in shibuya... its in there... you gotta look up when you get there...
Other shops i found paint at was the Haze store and the Recon store... both in Harajuku...
Japanese paint sux to do fillins with... just use it for tagging...
also go to a 100 yen store... they have mini cans there the size of large markers.... those are awesom for catching day time paint tags... and they're easy to rack but at 100yen... its nothing...