You wont be able to get a 68/45 steel tank, the steel ones only come rated 3000 psi. You would have to go fiber wrapped to get the 4500psi. As for getting a used tank somewhere, for the bigger tank I would look at PBN. As much as I hate to refer anyone there, especially anyone from here, it seems you can find in hydro 68/45 tanks for $100 or less.
Probability is a math law, not an aiming technique.