Your best bet might be ebay for this. I have a 51 cantilever build which was a balloon tire bike and a 53 cantilever I'm building right now which was a middleweight. I don't know exactly when the cantilever frame started, but I don't think it was for long before everything shifted over to middleweights. I have noticed very little difference in the two frames, both are solid and well made. The middleweight will accept 2.125 wheels with no problem. Fenders will be more snug on the fatter tires, but will also fit or if you're going with new fenders the bigger balloon ones will fit that frame OK. I mention this simply because I think there are a lot more of the middleweights out there since the cantilever frames became very popular as the straight bar frames faded way.
SB