Coker Tire lists 700-20 as available in highway tread pattern.
https://www.cokertire.com/tires/fire...ad-761401.html Not all trailed equipment got cross country tires. They didn't need driving traction and highway tires used less rubber.
If Coker has them the other usual suspects will too - Universal, Wallace Wade, Summit - whoever offers the best total deal on tir, tube, flap, shipping, etc.