I used to use International Tire and Tube in Texas, but they seem to have disapeared...
I noticed that my local Agri-Supply has bias ply 900x16 tubes in stock, I assume that the Southern States Co-Op (I got my 900x16 Grip Spurs there) does as well...
I have several good, used, 900x16 tubes in hand (put new tires on my XM708 before the Labor Day Trailrides and Rally in VA) that I could send your way... (or could bring since I have to pick up some stuff at Meade real soon...).