Check the following:
Ebay: Everything's at Ebay if you look hard enough, including some stuff you'll wish you hadn't looked for.
Salvage Yards: Depending on what state you're in, salvage yards may be able to sell complete totalled cars, most don't want to sell whole cars, but often will if it's something without a lot of demand (including 2.3 turbos sometimes, as most people just go with the 5.0)
Wholesale Dealerships: These guys carry the stuff new-car dealerships won't carry in their used department due to age, condition, mileage, or all of the above, sometimes you get lucky and find a slightly damaged fox-chassis (79-93) Mustang at one of these place, I've even found 2.3 turbo cars on them before, they seriously don't know what to do with the things, so they're usually sold cheaper than 5.0s and even V6 Mustangs.
Shady-Downtown-Type Car Lots (You'd be surprised what gems these rip-off artists are practically giving away sometimes while seriously overcharging for absolute junk, I DROVE my '84 Camaro V8 donor car with the rare 1986 hatch and third brake light, flowmaster exhaust, IROC rims (no center caps) a built TH350, and headers off of a lot for $500 because the thing had smashed fenders, a broken window, and a rebuilt title. The sleazeball that was trying to sell a lady a Chevy Citation for $3800 didn't realize what he had.)
Autotrader.com: You can search by year range, make, and model to find what you want. This is actually how I found my Camaro donor car, just looked to see what was there, hoping there was a cheap wrecked one with what I wanted. The cars you'll want to look for are: 1987-1988 Ford Thunderbird Turbocoupes and Mercury Cougar XR-7s, 1984-1986 Ford Mustang SVOs, and the 1985-1989 Merkur Xrt4i (here's what an XR4Ti is...
http://cgi.ebay.com/ebaymotors/89-ME...QQcmdZViewItem wish it was down here in Texas, I'd sell both my GTs just to have it!!!)
you can also post wanted ads in the classified sections of online forums such as this one
anybody know where 
