If Dangerous Dave, The Old Western Scrounger is still in business (I think he was bought-out by Navy Arms?), he'll have the ammo. Really, 7.7 and 8mm Nambu are not all that rare, and you may be able to find them at Midway. I found a box of Norma 7.7 at a gun store going-out-of-business sale, and older gunshops might be more likely to have it in stock.