I couldn't find any parts dealer in Kansas but I have a catalog from NPD from 2005 and they were selling their skins for $60.00 each. They are for 71/72 Mustangs but they say they can be easily modified to fit a 73.
Depending of what you need to fix you can also get a quarter repair panel:
Lower front section for $9.00
or
A section that extends 6" above flare and 40" long for $34.95
The full quarters or skins have to be shipped by truck but the repair sections can be shipped by UPS.
You can order or download their catalog on their site here:
http://www.npdlink.com/modules.php?name=Mustang_CatalogGood luck.