Eureka do some quite good Japanese in 15mm and 20mm that match up with their Australians (15 and 20mm) and their USMC ( 15mm). 20mm versions of the USMC are about to go into production.
Battle Honours do a range in 28mm but if you want cheap stand ins for play testing then why not avail yourself of some cheap 1/72 plastics? Airfix, Matchbox, Esci and Revell all do WW2 Japanese.