Welcome to the site! Join us in the II specific forums and be sure to post a pic or 2 - I'll say that now because you'll be asked anyway!
It's been a hot minute since I had my car's body work done (back in 2001....) and I'm originally from Waterloo. Seems like I ordered my quarter skins from Keystone body supply? But there are a plethora of places online that offer them, apparently it's one body piece that's still available....
Buy Sherman Parts 471-50L at JEGS: Sherman Parts Quarter Panel Skin 1975-78 Mustang II. Guaranteed lowest price!
www.jegs.com
Reproduction of the original style quarter panel repair skin designed for use on 1974-78 Ford Mustang II models. Each quarter panel skin is manufactured to factory specifications featuring the correct recessed area on the front side of the panel, pre-cut side marker lamp opening and fuel door...
www.classicindustries.com
1974-1978 Mustang II Reproduction Quarter Panel. Fits 75-78 and 74 with use of 75-78 filler. These measure 26" at the front and are a perfect fit. This item may ship via freight, costs vary by location. A default shipping cost is applied, actual costs will be determined and customer contacted...
classicautoreproductions.com
Now if you're talking about the inner, that'll most likely need to be fabbed.