Contact Skat-Trak, they can put paddles on your supplied tire carcass, and probably have complete tires also.
One question though: If you are going to ride a WR-X in the sand, what are you planning on using for a front tire?
Skat-Trak's info:
P.O. Box 518, 654 Avenue K, Calimesa, CA 92320 U.S.A. • Techline 909-795-2505
Fax 909-795-6351 • 800-969-7528 (orders only) Business Hours: 7:30am - 5:00pm PST
Web Site: www.skat-trak.com • E-mail: info@skat-trak.com