[That is a much easier question to answer than he'd been fearing. He would answer if she asked, but probably better not to dive into the whole Captain America and Tony Stark issue right now.]

Ammunition comes up sometimes in the item shop. Otherwise, you can earn points from doing missions for the Malnosso. You can use those points to buy ammo you need. If you're just wanting to keep up with marksmanship, you're as well using the computers in the Battle Dome to simulate whatever weapon you need rather than using your own ammo.

Otherwise, might want to pick up a weapon that doesn't use ammo.