Just for the sake of introducing a contrary note, I bought a street tire for my KLR in Granada Nicaragua when I found it was the only 17 inch tire available. In addition to the inappropriate tread, it's also too narrow. It's kind've skittish off the pavement, but fine otherwise. I expect I'll wear it out pretty quickly, then see what's next.
Seems like if you want to go to capital cities and/or wait a day or two, you can find tires for the KLR. If you don't like to plan ahead much, and hate shopping in large cities, 18 inch tires are easier to find.
OTOH, there's a KLR dealer just a block from where I'm staying in Cali with a 2009 KLR 650 in the window. And I saw a KLR 250 in a dealership in some otherwise-unremarkable little burg in southern Costa Rica. So it's not like such things are unheard of--it just takes a bit of planning, patience and/or intelligence....all of which I generally lack. And for what it's worth, Colombia seems to be the best place to buy parts this side of southern California.
Hope that helps