You can make your own for 10 percent of what you buy a patented unit for.
Go to big box hardware store or shop the web for heavy duty swivel wheels that have ball bearing swivels. If your garage floor is not smooth buy the hard rubber wheels...if the floor is smooth and uncracked, buy the all steel wheels...they roll around easier.
Then buy two treated 2x6x 8 lumber and make a "cross" with a strong bolt to attach the two pieces so that the cross can be swung back and forth ....position the whole deal between the struts and swing under each ski. Cut the board that will stretch between the skis just a couple inches wider than the width of the sled. Then lift the rear and slide the longer plank under the track.,,,
The whole sled is now on rollers. I put some non-slip rubber (old lawn mower tire cut up) to position on each ski section so it won't tend to slide off the plank...and another under the whole length of the plank under the track. (well not the whole length...but a big section...have to be able to scissor the two boards and if too long it will impede the scissor action
If you are careful, you can start the sled and goose it and the whole deal will roll out the back of the sled....but if you hit the throttle too hard it will go flying.
Probably 40 bucks in parts....when not in use, it folds up nicely along the wall in the garage.