K.I.
My oldest nephew and his Dad both wear a size 13. My nephew has been that size since he was in the 8th grade.
Size 13 is relatively common and easy to find. it is when you get bigger than that size 14+ when it comes hard to find and you have to custom order.
Although while looking for my nephew (whom is now 17!) over the years, I have found that there are a great many stores that sell shoes all the way up to size 15, Nordstroms rack being one of them, just FYI!