Mustang Parts House is at
www.mustangpartshouse.com. They are local for me, so I've done on-line as well as in person buying from them.
What I do like about them is that they have a very liberal return policy. I've on occasion (maybe more than that) bought the wrong part or one I really didn't need. They either refund or apply to my next purchase without many questions or hassel.
The front valance I bought was sheet metal not fiberglass. I might have just been lucky here, but it fit better than the one I took off.