a tire with an agressive tread patter like the 4 wheeler tire posted earlier is going to have alot of rolling resistance especially in soft stuff like sand.
I would looke for a tire with little to no tread, or something that isnt going to fight the terrain. maybe some mower tires if you cant find any smooth ones.
I just found these, and they seem ok. they have very subtle treads, are fairly cheap, have bearings, and are 16 inches tall.
http://cgi.ebay.com/4-Wheelbarrow-Ca...3A1%7C294%3A50