How to get sulfate reducing bacteria?
Sulfate-reducing bacteria are everywhere, one spoon of soil contains several different species of them. The problem is how to isolate them from the other bacteria.
This old paper explains how to do it, I am sure there are newer techniques to do the same, it's up to you to dig into the literature to find the one you prefer if you want to isolate them by yourself.

Otherwise, just buy it from a collection.