I bought a fairly large sheet of the orange-ish Uni stuff years ago. I'm not sure which is which but I think it's color coded (red, yellow, greenish blue etc.) according to density. At the time the orange-ish stuff apeared to be about the same density as stock, but I bought it at a shop where I could compare them.
I don't know if ProCycle deals in it, but if they do maybe they could advise you on the various densities.
I cut two exact copies of the O.E.M. filters out of it and we're still using them today.