rjlope, I see this is your first post, so welcome to our 71-73 Mustang world!
The park lamp/turn signal lens was bonded to lamp body from Ford and was not serviced separately. If your lens has just come loose and not broken you may be able to bond it back on with a good quality plastic glue. If broken you can source one from a Mustang parts vendor (we have several on here) or search E-Bay for D3ZZ-13200-A which is the Ford part # for the lamp assy.