Charley's has them. They're located in Mt. Vernon, so a stop on your way down or back home may be in order. Charley's is also a regular vendor at the F&G show with a large booth in the commercial area. They may even have them available for sale at the show.

And there are several other local greenhouse manufacturers or suppliers that participate in the show as well.

Here is a link that might be useful: Charley's Greenhouse and Supply

If it the same as heating cables for pipes, any hardware store should have them.