Ammo, is ammo, is ammo. Pure ammo just makes it really easy.
It takes longer, but raw shrimp, fish food, anything that rots will work (i don't recommend meat though). As it's rotting it continues to generate ammo and you need to watch that it doesn't get high, or if it's been in for a while that it doesn't foul the water too badly. I used left over peas on a tank the had minimal seeded material and it worked just fine. No matter what you use, put it in a stocking (washed/rinsed with hot water) or fine mesh bag.