I found ammo for the 7.7 Arisaka in a number of places.
I have some resized 30-06 ammo that I got for $1 per round from a reloader friend.
I also have some WW2 ammo for display that I got for $1.50 per round that I bought off of gunbroker.
I bought some new at a gunshow for $1 per round (graf cased).
It isn't common but I usually can find some somewhere.
Reloading seems to me to be the way to go with this round if you shoot it in great numbers.