Since you are in La Paz and questioning a few hundred km more, why not just purchase there? (However, you will not be able to buy anything tomorrow as it's census day in Bolivia.) On Colombia (near Boqueron) there is a street filled with tire options. See these posts for more info:
(scroll down 'till you see the Simpsons reference...)
I am currently in Cochabamba and can ask around for options, but it may have to wait until Thursday. Let me know if that would help...