2008 quad cab tonneau cover

I have a 2008 Dakota Quad Cab with the cargo rail system on the bed. My vinyl roll up tonnaeu cover that I used on a 2006 quad cab wont work on the 2008 w/cargo rail system. Can anyone direct me to where I can get an inexpensive roll up cover for the 2008 w/cargo rail system? I checked the dealer and they want $400! I only paid $100 for the cover for the 2006...