It says on their website to call to order.I have only shot the bullets it came with but have read some of the same material you have concerning projectiles, need to find something else that will work because they seem to be out of bullets at the NAA website and of course no bullet equates to it just being a neat little paper weight! I did see a YT video where some guy converted his into 22lr and they may sell the conversion kit but don't hold me to that.
I think number for buckshot and the 22 air gun slugs are your best bet other than the factory made ammo.