Where can I get a cover for the outdoor wok?
You can use your wok upside down on the burner such that no dirt can get into the burner holes.
Convenient enough, flipping the wok upside down after cooking serves the purpose.
I was thinking more protecting it from rain, dirt and sand.
How about a canvas drop cloth like http://www.homedepot.com/p/Everbilt-5-3-4-ft-x-8-3-4-ft-Canvas-Drop-Cloth-69CV8/202029402? This can be found in any local hardware store.