Melanie - I purchased the gasket directly from Scamp
and had the new Lexan windows
(front and rear on my 1983 Scamp) installed at an automobile windshield replacement business. The business carried an identical gasket, and it was priced similar to Scamp's, so you might be able to purchase one locally and save shipping time and expense.
There is also an inexpensive tool which is recommended to help with the job. If you do a search in the forum, you'll come up with several threads relating to the replacement and the exact tool.
I'll give you a heads up that the windows
can easily still leak after replacement, so do your research for additional gasket sealers, etc.to be applied in the gasket before you start. I've had mine back to the shop twice and they still leak some.