Actually, forgive the ignorance or maybe I missed a point here - but, what's the problem with asking them to come in and sign again? If the person refuses, then make a report. Also, do you not provide them with a copy? If so, ask for their copy & see how that goes.
If they refuse to come back in and sign it - remember to keep the pen they used (in case we get all technical & need forensics) - hopefully you had the same sheet under the original form (and they pressed hard enough to trace their signature on the bottom copy) and then go make a police report for robbery I guess - stealing legal documents should be illegal right? Do you have proof that you paid them? Did you film that part?
Ok - I have too much legal influence and watch too many shows, I know
Good luck though - hopefully those will spark some other ideas on how to get your 2257 back - whether a new copy or the originals.