Do you have concrete floors? I use the Harbor Freight mover's dolly wheels because I can easily move my sled laterally. I also use the Sure Grip wheels to drive on because they stay on and hold up better. I can also use them to drive up the road to get to snow if I have to (although not on dirt).
If you have concrete floors the one's Arctic Cat sells in their Accessories Catalog work the best out of the one's I have tried. http://www.arcticcat.com/snow/accessories.asp
The Cat Caddy's on page 134.
build your own you can get the best casters for a good price and get some plywood cut it how you want even double up the plywood with a split in the middle for the skag the possibilities are endless. And you can paint them your own color to make them look perty