I found a transfer switch on e-bay a few years ago, but found Camping World cheaper.
First I went to Camping World's web site to get the specifics and any options or choices, as well as prices and then used the nomenclature to try and find sources for a switch. E-Bay turned out to be the only other good resource, but prices there were not any better than Camping World. My old switch was a manual type (1993), and the new one was electronic. Works great. (I have a 50 amp system, with a 10 KW generator.)