It was purchased at a local truck service center. It has a replaceable element, which I have replaced twice. The original cost was, I think, (remember this was over 20 years ago) about $20, or maybe a little less.
It was placed between the fuel tank and the prefilter in the engine compartment. All it took was a little extra fuel hose to go from the metal fuel line coming from the tank to the separator and then from the separator to the prefilter. The biggest problem was finding a suitable place to mount it. I finally attached it to the frame.