The Mark 1 uses a different docking sled. A lot of people are searching for a Mark 1 sled, so if you find one available, post where it is - others are searching for them!

The Mark 2 and RioCar (Mark 2a) use the same sleds. You can get an extra sled directly from the Rio store. I have not really seen any for sale used...

I have not seen many used sleds available at all.