Hi John,
I would suggest buying an entire parts car for the following reasons,
1. Cost of transporting a car may be equal or even less than shipping just a roof.
2. Less risk of damage if the roof is attached to a car.
3. You have the option of cutting it off the car where you want it cut and not risk something cutting it wrong or damaging it.
4. All of the extra parts can be sold off or used on your car. Depending on how this works out, you could come out on top and make a little money.
There are many decent donors out there and of course the donor doesn't need to be a GS. My experience has been that cars from the rust belt rot from the ground up and cars from the South or West tend to rot from the roof down. This might be important if you are just looking for a good roof and a rust belt car could work.
Best of luck with your search.
Chris
