You've got a '73? My experience is with '65 parts, but the 3 companies I use all advertise they supply '73 parts also. First, of course, there is eBay. I've had moderate success with my Mustang purchases, good success with other items on eBay.
Of the companies, I like Mustangs Plus (www.mustangsplus.com
) (800) 999-4289, best. Their 90 day return policy is the best I've found. They run sales often, so I made a "need" list and a "wish" list and check the sales flyers against my 2 lists. I try to only buy when the item is on sale. Mustangs Plus also has the most comprehensive and useable index and the catalogue is filled with practical and useful information on the parts rather than just a catalogue of parts, period. Sales people really seem to know Mustangs themselves which is helpful when you want to know details about a particular part.They are VERY prompt. Shipping costs are reasonable, but they are in CA and so am I.
Next, I equally like Larry's (www.larrystbird.com
) (800) 854-0393 and California Mustang (www.cal-mustang.com
) (800)775-0101. I've been a customer of CA Mustangs for about 8 years and am a relatively new customer with Larry's.
Never had any problems with any of these three and they have helped me a lot, let me return long after the stated return period, let me exchange on a bunch of interior parts when I decided to change my interior color, and have always been very pleasant.
(I do not, nor have I ever worked for any of these companies, tho wish I did so I could get the employee discounts!)
I have not tried the used-parts-on line from-dealers route.
You might ask if they have an active military duty discount. Given the times, maybe they'll just make one up if they don't have one! Good luck! :-D