I'm not even sure I can find a assassin snail around here. I don't think i can get a dwarf puffer with my fish.
ask at your LFS most people have them
I bought mine at PETCO they are not listed on the tanks but if you ask them they will probably have them scattered throughout the tanks
This is what your looking for they are aprox $2-$3
or you can find them on Aquabid.com or Ebay but with shipping it will cost a little more