Here you can buy the wheels seperate if your making your own ...
Hobie Kayak Wheels - Sports & Outdoors - Compare Prices, Reviews and Buy at Nextag - Price - Review
If you do make your own ,,nice to make them so they fit in the scuppers like the Hobies do ..
These are the ones I bought ,,, not for soft sand but I walked my kayak about a mile on a cranberry bog road with these .. like walking the dog ..
http://www.austinkayak.com/products/...t-Plug-In.html