Gardening area of hardware stores. Nurseries. Wal-fart even.
I'm telling everyone for the last time: In the good ol' US of A (forgive me, but I am assuming you are a US native. If I am incorrect, feel free to flame) In the USA, mono-ingredient fertilizers are no longer sold in brand name stores like walfart et al. They are always mixes of all kinds of stuff.
Finding ammonium sulfate is easy. Just look for pure ammonium nitrate, and the sulfate appears everywhere. Worked for me when I wanted nitrate for you know what once upon a time and it was nowhere to be found, only sulfate sulfate sulfate.
Nowadays, phosphates are pretty popular, too.
Why do you need the sulfate?
PrimoPyro
Vivent Longtemps la Ruche!