If you can't find or don't want to use old used tires they are reproduced by Specialty Tires of America.
http://www.stausaonline.com/military/ndt.html
The tires made by STA then get sold by:
Wallace Wade
http://www.wallacewade.com/
Universal Tirehttp://www.universaltire.com/truck-and-military/military.html
Coker Tires
http://store.cokertire.com/tire-styl...?adjclear=true
Not all retailers list the 1200x20 but STA list them as a current product