It depends on how much weight you're willing to move around. If you want ridiculously overbuiit, you can get a wheelset built up with the 48 spoke version of these:
http://www.velocityusa.com/default.asp?contentID=700

And when it gets cold you can put some fatty studded monster tires on them.

Otherwise, just get some 36-spoke wheels with wide rims.
http://www.velomine.com/index.php?ma...roducts_id=336
Something like that. I have no idea if that's a good store; it was just one of the first Google results for "fixed CR18".