It may be tough to find boxes of 7.7 ammo. Nearly all of the ammo captured in the islands during WW II was destroyed. Nearly all of the ammo captured in Japan after the war was dumped into the ocean. The ammo in the hands of the Japanese army in China was captured by the Chinese and used up the the civil war and in the Korean war. Except for some Chinese production for the Japanese rifles in their hands, no one else has manufactured military ammo in the Japanese calibers.
The other major WW II rifle rounds; .30-06, .303 Brit, 8mm Mauser and 7.62x54 etc. were used in post war conflicts in Africa, the Middle East, Southern Asia Yugoslavia and others. A LOT of ammo was made for all of this and some has ended up here on the surplus market. The Soviet Union and Warsaw Pact had millions of Mosins and captured Mausers in inventory and stockpiled ammo for them. A lot of the rifles and ammo has arrived on the surplus market as well.
Compared to the other WWII rounds, there was never much surplus ammo available. Your best bet may be to do a search on vendors of collector's cartridges.
Drue