I confirmed with Thule that, unfortunately, the pads of # THKIT3131 cannot be purchased independently from the rest of the hardware. view full answer...
The Saris Superclamp EX part # SA4025F that you referenced does come with 4 wheel trays that are wider to allow fat tires to fit on the rack. I attached a picture to this page where you can see all 4 of them mounted to the bike rack.
The max tire width this rack can accommodate is 4.5 inches also. view full answer...