If you can't find a low mile one on Ebay, then check out car-part.com. That's how I found my tranny. Also, you can use the trans. from a Mercury Grand Marquis, a Vic (as long as it's not a police or taxi) as well. Hate to see you drop more money for a trans just because it came from a 'stang.
I'd get the 01 trans, have the spiral ring done by Alan, j-mod, Mark VIII converter and flexplate, add a deep pan w/ drain plug, and make sure to use a nice stacked-plate cooler.
If you get lucky, you may be able to keep this all down to around $800 if you do all the work yourself. Figure $500 for a trans, $200 for TC and flexplate, and another $100 or so for the Mercon V, gaskets, and the little things along the way.
Good luck on whatever you decide. I just think that it would be easier now to build one up and have it ready to go, instead of having the trans. die and leave you stranded somewhere. Would cut out some of the downtime when the replacement comes due.
These are highly engineered precision vehicles, the first step in diagnosing the problem is to strike the suspected offending part sharply and repeatedly with a blunt object, then re-test.