Currently there is nobody making decals for these planes. I searched and searched for just about anything I could find before a friend came to my rescue with a portion of a Cutting Edge sheet. To my knowledge Cutting Edge is the only company to ever make after market decals for the SR-71 in 1/48th. They made a whole range of sheets and I think one could model nearly every one of the planes currently in museums including the NASA birds and many other more obscure planes.
Testors has reissued the 48th kits in the past so you may want to give them a try for a replacement sheet though I've not heard of much luck there. Ebay will sometimes have a sheet of the cutting edge stuff show up though in my three weeks of looking I never came across a set. I'm still looking for the CE resin engine set for the kit to no avail. Even after installing the kit parts, if I came across the set I'd rip out the kit parts in a New York minute.
I just recently started my own 20 year old Testors kit and boy is she a peach.
Good luck with your search.