These guns are shipped from the factory with the bullets that are ordinarily loaded in .22 short cartridges. Acquiring a mold ay prove difficult and would have to be a custom mold I reckon.

However, there are probably a couple of options. See if you can get an ammo company to sell you just the bullets or have you thought about "round ball"? #4 buckshot is .24 calibre and may go quite nicely....

Failing either of he above, the bullets are availibe from NAA direct.