Every critter which can shoot something will give you its shooting ability if you eat it. You can stock a nice pile of different shots this way. 
And even some that don't shoot give you shots, like those triangular green things.
Moneyes. And according to eats.txt the nests give you eight shots of them and the moneyes themselves, naturally, give you one. They do four damage, same max speed, and can bounce.
@Pelar: Spicy Soup. Gives you 24 Sea Dragon shots. They do the same damage as Moneye heads, apparently.