CR,
There is still ammo available for both type 38 (6.5mm) and type 99 (7.7mm) from Norma. Norma also sells the unloaded, unprimed casings. I've yet to see anything in the way of surplus ammo for these rifles, I keep an Eagle eye peeled in Shotgun News. I'd be suspicious of it anyway - if any were made available. I don't know too much about the makeup of the wartime Jap ammo, but I would bet that it was corrosive in nature. Not that this is a really bad thing, it's just a hassle to clean the weapon after firing it, and if you aren't dilligent enough, you have a problem on your hands.
If you want to mess with an Arisaka, then get the Norma ammo, and when you've smoked enough rounds downrange, get the RCBS dies and reload the suckers!
But to echo what the previous poster said, there aren't enough similarities between the 303 British and the 7.7mm Arisaka to effect an easy conversion.
Good luck, Unkel Gilbey