Mustang boards, the for sale forum here, any mod motor site, the crown vic sites. I see them all the time on the
www.crownvic.net site. I personally think a used Ford/Motorcraft OEM is better than the aftermarket, BUT that is a complete set with everything you need and might not think about, like the alternator bracket, bushings or new water neck, etc. The Ford/Motorcraft swap kit is pretty extensive and expensive.
I bought mine from someone on this board. It wasn't in perfect condition and looked like it was upside down in a swamp and stuff like the thermostat was welded in place and it was oxidized, but it cleaned up and a little more work to get the thermostat out. I live in the desert southwest and I am just not used to rust and oxidation, but the last few things I have bought used sight unseen from the east coast lets me know what they go through!