Cheapest...A junk yard. I can get one for $15.00....If I can nab it before someone else does. Almost had my chance a few months ago but someone beat me to it by a day!
The most credible and available place to get it? MustangII.net, but good luck finding someone who will let go of it. Probably won't be very cheap either.
These Stang II pans are worth their weight in gold when it comes to hot rodding a Pinto or restoring a Mustang II, no joke about it.
Don't forget to get the oil pump pickup tube if/when you get the pan. I have heard they are a matched set (please correct me if I am wrong).