That is a Blue G12 fiberglass coupler. Madcow is currently out of stock, but I will buy one from them should their stock be replenished. I have other rockets to build in the meantime, but if you find you have a blue G12 4” x 8” coupler in your parts pile and want to sell it . . .
Wildman, I believe sells blue fiberglass-call them, usually the couplers between Wildman and Madcow are interchangeable. Alternatively, if you use a common uncolored (light green) coupler, it won't show through the colored body tube.