I had a really nifty one I made from motorcycle wheels, which have the bearings built in. I got them from a junk yard, and they were a matched set of front wheels with tires from what I think were Honda CB400 street bikes. The large diameter wheels are really nice for rough terrain or deep sawdust. Mine did double duty as a small trailer I could pull behind the truck, which also made it convenient to bring to job sites. That's how it ultimately met its demise, though (but not my fault :)). CB.



Meanwhile, you can get a lighter-duty one here:
http://www.timbercarts.com/

Or you could order a set of wheels and build your own:
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200331764_200331764