Are you talking about source files for gmax?

IF you have to pay ANYTHING for an (available)  source file, you've paid too much.

There are gmax source files available at Freeflight Design Shop

Also, using the CVA converter, you can convert FSDS source files into dxf format that can be read directly into gmax.

Good luck, and as usual, respect the original builders'  credits and copyrights.