Can anyone tell me where I can find "Tube only short distance season
ticket" prices?

London Travel Information (020 7222 1234) couldn't tell me this and
told me to go to the tube station - which seems a bit lazy on their
behalf!

I think they've given you the correct answer. It seems if you want to find
out about them you have to go and ask at a tube station. They might at
least give a few more clues as what journeys they are available for,
instead of just saying some short ones across zone boundaries and way out in
Metroland.