My experience had me checking in with Don at OMS (Ohio Mustang Supply) - see what he's got. He only stocks the good stuff, and if he doesn't have it or can't get it, he'll let you know.
My back-up plan was then a toss-up between National Parts Depot and CJ Pony Parts - depending on who had what (one would have this part, but not that one, and the other would have what the other didn't, and so forth).
Honestly, you really can't go wrong with any of the vendors we have listed here - they're all top-notch. I just like that with OMS, you're an actual person and not just a customer number - plus Don spends a lot of time here, so you will more often than not get an answer straight from him.
What are your plans for the car? Faithful restoration? Restomod? Are you hoping to use as many OEM or original pieces/parts as possible, or will reproduction (repop) get you where you want to be? Repop parts usually take a little bit of adjustment to fit just right.
I would have to say that mine is now somewhere around 50% repop sheet metal (fenders, f/r valances, battery tray & apron, quarter panels, floor pans, trunk pan & drop-offs, taillight panel, and trunk lid). My interior is also pretty much 80-85% reproduction pieces - I'm kind of in the middle of Nowhere, Texas, so it's all pretty much mail-order, since there are very few salvageable '71-'73s anywhere near me.
Hope this helps!