I have purchased it from ebay in the past but these days it seems really pricey. Does anyone know a place to get it at a reasonable price or even a decnt price seller on ebay?
Try a lighting shop. Lights in commercial buildings are often covered by plastic egg crate that is dirt cheap to buy. One made for a large commercial fixture would probably be enough to take care of several typical tank's needs. In the US it would sell for $2 or $3.
It is used with sharp objects like large rocks to spread the weight across large areas of the bottom so that you don't create stress cracks in the bottom glass Aaron.