Well, if you don't want a young or small one, you already have your answer! Buy an older one
Personally, except for the odd number out, I've only ever seen hatchlings and yearlings at expos! If you don't want a young one, yearling doesn't sound half bad, does it?
If you aren't looking for a particular morph, you could adopt a rescue. Rescue organizations and maybe even regular animal shelters have lots of ball pythons that have been rehabilitated and given a clean bill of health, and are looking for good homes.