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. Please Contact for Availability. Currently both sides are in stock but availability can change on short notice
classicautoreproductions.com
Now if you're talking about the inner, that'll most likely need to be fabbed.