Oh thanks for the info logiclee. There are some manufactured ones around on the internet for much cheaper (about $10-20 USD), but most seem to be made of styrofoam so could possibly float around if not glued in. The resin is probably what makes the one on E-bay so pricey, it's not made of 95% air like styrofoam is...
The seaview stuff is good, but olive oil is too I've used both with comparable results. Because background is generally sold in rolls, you may have to tape the edges down too if the side you use was the outside of the roll.