I don't know what size you're looking for here. I have mine made by a local gasket & rubber company. I had to pay $100.00 for each die size. Now, I just pick up the phone and order a bunch. UPS gets them here in a day or two.
I have several hundred on hand, right now. My size closed WFC is 5" x 13" - outside by 4" x 12" inside Material - BunaN or Neo
My meduim size WFC measures 5" x 9" outside by 4" x 8" inside
Let me know at: randy@hhydrogas.com
Cost? Depends on how many you want and how many I have in inventory.
Cheers,
HHO-ing in N. GA