can try asking around here:
http://www.pcenginefx.com/forums/
http://www.pcengine.co.uk/
prices vary to be honest.. I've ordered stuff from this place too but paying them is a little tricky.. you can set up an account and do a test run!
http://www.japangamestock.com/videogames,jeuxvideos,import,67.html
also I've used these guys once or twice for stuff.. just remember about import tax tho..
http://www.play-asia.com/paOS-14-71-c-49-en.html
Other than that compare the prices on ebay. and please please PLOX I know this sounds patronising but always run spy checker before you order stuff!!