Specialty Tires of America is a manufacturer and as far as I know doesn't sell direct.
http://www.stausaonline.com/ They sell through many of the restoration and speed equipment suppliers.
I also bought my 10.50x16s from Wallace Wade
http://www.wallacewade.com/ although the website seems down at the moment.
https://www.cokertire.com/tires/styl...and-truck.html (Coker may suggest that tires made by STA are "Coker" tires)
https://www.universaltire.com/truck-.../military.html
http://www.lucasclassictires.com/Mil...k-Tires_c6.htm
http://www.summitracing.com/int/sear...illocation=int
http://www.jegs.com/webapp/wcs/store...=military+tire
and probably many more. Pricing and shipping price/policy can vary considerably, research for your needs and location is suggested.
I installed my own 10.50x16 tires and am by no means an expert tire installer.