I bought mine straight from Ford. A little over 100 bucks but worth the extra cost so you're not shooting yourself in the foot with a POS discount store IAC. I figured mine lasted over 100K miles, so it'll be worth it if this one does too.
I had an IAC go bad, so I made the mistake of buying a POS autozone replacement and it was already a broke ass pile of crap when I put it on. I went to the junkyard, and found 3 for 8 bucks a piece, one of which was brand new OEM Ford replacement.
Otherwise, save your time and just me the money you'll be wasting on some POS aftermarket part. Mustangs Unlimited is very good about having FORD stuff in stock and being able to ship it out the same day (?order before 2pm?). Also, check other companies.
Note: Buy FORD ONLY IACs and EGRs because they are subjected to a lot carbon from the EGR system. Buy a FORD TPS because it's NOT easy to make a weather-proof TPS that doesn't get flat spots. Buy a FORD TFI because it's subjected to a LOT of heat and vibration - the after market POS units have a tendency to doe quickly.
It's *your* money. All I can do is tell you how to spend it wisely.