Try these guys: http://www.satolabeling.com/  Or these guy's: http://www.horticulturalprinters.com/login/login.aspx  There is another one, 8 1/2 X 11 but not sure it they supply an outdoor version, what you need is "thermal" lables, the first link has some Thermal presure sensitive signs, mostly intended for nursery stock, they might have some for your laser printer, but other wise I think you can use a special marker.

Hope this helps.

Aaron.