I heard that Joel at Baldwin Motion charges $1500 thereabouts for a converted car document but that I can see, and would pay if I owned one, being they are so rare and specialized cars.
Depending on my mood at the time, if a dealer said he had an invoice copy for my LOS L35 car I would make him work for that $$$$ and request at least a picture showing the paper with the VIN, as a teaser, and he can cover the other line items. That can be done in a matter of minutes on his part. If he wants the money, which he will definitely pocket, then maybe he will do at least that. Tell him for that amount of money he has to show you something beforehand.
Or, try the old school Brooklyn method and
make him an offer he can't refuse (past the pasta) and send Rocco and Joey to the dealership and whisper to the guy that it will cost more than $2,000 to replace the showroom glass if some vandal with a pellet gun happens to pass by one night
I don't have a horses head laying around but I do have a 396 head you can borrow and put in the guys bed one night while he is sleeping.
All kidding aside, If you really wanted that invoice you can negotiate because both sides want what the other has, $$$$$ & invoice.
Mike