The Tomioka Low Down Slopes are pretty nice and light weight, just kind of expensive.
NRG makes the "slope box" and it looks a little bit higher, and it goes for around $45. The only thing is I can't tell if its sold singly or as a pair!
Another thing that concerns me about the NRG is that its completely flat..so if you're not careful, you could drive right off the edge. At least the Tomioka ones have a dip for the wheel to sit.