I got a set or 16.00x20 for my Bucyrus Erie scraper rear tires, from a surplus tire dealer in Leavenworth, KS but danged if I can remember the name of the place, been 6 years now. I got michelen XL series 16.00r20 and they are said to be a good tire, the reputation of the 14.00x20's is a little less stellar. I was told they dry rotted much faster and had a shorter service life, they are also harder to find. I think the tires that replaced them, the XZL series would be a better tire to look at if I remember correctly Pete, hope that helps.
Tiretown.com
Do a google search for surplus tires in Leavenworth KS. I did when I first read this and then couldn't get on here to tell you. Finally got on here and now don't remember the name. catskinner
gensco has government surplus tires