If you mean the pieces of outdoor carpet held on by springs, you can find them at WalMart. With the four step system, however, you can't put them on all of the steps because the springs will interfere with folding them up. I've got one on the bottom and third step. But, now that the light comes on, I probably could put them on the other two if I used cable ties in place of the springs. Once they're on, they dont really need to come off.
On our previous TT, I made some from pieces of outdoor carpet I had and attached grommets to opposite sides. Those I held on with cable ties. Worked pretty good.