Can I get a replacement cushion?

Remove seat track plastic covers. Remove seat bolts

It is possible - on most cars - to replace the seat cushion only; I would suggest to buy a complete seat to avoid different shades of fabric and because it is a lot less involved to change the whole seat.

On what? 3 possible ways the rear seat cushion is removed. On some the seat has clips that lift straight up. Or some have 2 small bolts at front bottom of rear cushion. Remove the bolts and seat slides out easily. The final way is old style: a heavy wire on bottom of cushion, one on each side, fits into slots that come up from the frame underneath. To remove, push cushion hard to the rear until the heavy wires can be removed (lifted up) from the slots. Do one side at a time. These are the most difficult to remove the rear bottom cushion. You have to push hard against the cushion near the bottom of it, pushing it backwards, so the wire comes out of the slot, and then cushion can be removed.

First try lifting straight up on the cushion. Since there is an access panel to the gas tank, it is very possible the cushion is easily removed by lifting straight up.

Have the seat re-done, or buy a cushion to sit on.

Reach down in the bite-line (where the seat cushion and the seat back meet). Pull up on the seat cushion until the cushion raises above the bottom of the seat back. Pull the seat cushion towards the front. There are two metal tabs that are hooked into the front of the seat cushion that holds it in place; unhook these by moving the cushion forward. To replace, reverse the procedure, but make sure the seatbelt recepticles are exposed before pushing the seat cushion down into the bite-line.

depending on if it is the seat bottom or the seat back as they are seperate systems if both dont work then you probably have a electrical problem with the module or the switch not the heaters them selves, if it is one of the heaters then you must remove the seat cushion and replace it or the seat back cushion the heaters are intergrated into the foam the seat cushion unbolts with two fifteen millimeter nuts just under the front of the seat and unclips from the rear as you pull up and forward the seat back cushion is much more involved and should be done by a pro

Ebay

According the customer support at Dorel, there will be new seat liners/hoods for sale soon. 

OK. On the front edge of the seat bottom cushion, pull up hard, both sides. This will dislodge the 2 clamps that holds the seat cushion to the floor. Pull the seat cushion out & set it aside. You will now see the female latches bolted to the area where the rear of the seat cushion meets the rear seat back bottom. Remove the bolts (14 mm I think).