heater core - where to buy

Ford OEM. The largest majority of folks who've swapped them have had better luck with the Ford part.

Make sure you remove ALL the old sticky-goo from the air box before you intall the new core.
 
I'd go get the closest parts store one. Carquest in my case mostly. I have had a Ford unit fail in all 3 of my Foxes. And one aftermarket unit. Does not seem to matter, I think the tube and core size, combined with a hot/cold environment, vibration and chemicals just kills the little suckers.
 
Get one from the Ford dealer.

I replaced my heater core 2 weeks ago...(f`n major pita btw).

I last did the job 8 years ago using a NAPA heater core.

The Ford heater core has thicker tanks and thicker inlet/outlet pipes compared to the old NAPA heater core.