Do most touring bikes have a 135 mm rear dropout? If so, can you get by with a 130 mm rear hub or do you need a 135 mm rear hub?
A related question is whether most road bikes have 130 mm rear dropouts?
A friend of mine on tour on the Baja peninsula of Mexico needed to replace his rear hub (on his 2 month old Cannondale T2000) and was unable to find one there. I was surprised how difficult it was to locate a 135 mm rear Shimano compatible hub online. The only place I could find one in the U.S. was Peter White Cycles.
That's the standard MTB size, so there are literally hundreds of choices online or in any local shop that has MTB. If you have an old bike with 130, that's fine, though you will be looking for stuff like freewheels, in which there is not huge choice. On the other hand, if you are getting a new bike 135 is the way to go. some go 145, which if nothing else is a vote of confidence in the wider 135 spacing.