Climbing shoes can be stretched, and you are correct in that the rubber will not stretch, but the leather or synthetic will.
Leather shoes will stretch more than Synthetics, and any lining in the boots will reduce the stretch. At best out of a leather boot, you'll get a size, probably less. Synthetics about a half size.
This will al depend on shoe model, construction and how much they have already stretched.
A shoe repairer will be able to stretch them for around $10, Araps Resoles and Big Johns are a little dearer as they include postage charges.
Plan B - soak them in water, preferably luke warm but NOT hot (sole glues are heat sensitive) and then wear them till they dry out. i.e. watch TV for a few hours wearing them etc. The catch to this is that they will give and set to the shape of the foot shoved in them, so if you take your feet out and they dry with little bumps here or ther, that will be the new shape of your shoes!
Give either a try.
Cheers,
Tim
Cliffhanger Climbing Gym |