Mamboitaliano-
You might consider online shopping with some of the parts suppliers you find on the "Links" section of Stovebolt. Chances are my local auto parts store would sell the tube fittings that 3B mentions, but I doubt they would sell the gage itself.
Choose any supplier you like, but I saw that American Classic (a Stovebolt supporter) has both the gage and the tube/fitting kit
on their webpage. I also recommend checking the
Stovebolt forum users glovebox manual for new user info. Also, don't forget to use the
search feature. Out of 100 questions I have, I bet 95 of them have been asked by someone already. It is convenient to read all of the inputs without having to wait for folks to reply to you.
Good luck with your conversion!
Bill