...based on my own experience, were I looking for a project, I would not limit myself to the 1950's. A lot of the stuff from the 1960's with cottered cranks is very similar in geometry and componentry. And it gives you another decade of candidates that are available. These two I have were just accidental acquisitions, that came up on the local CL over the years. I guess if you're open to ebay and other sources, you might find more candidate bikes. The people over on the CABE have a lot of 50's stuff for sale, but it's mostly fat tyre Schwinn's and Rollfast's.
Also, if you do buy something from the 50's, and want it to be mostly period correct, look for something that still has all the components on it. Some of that stuff is hard to find, and you end up on ebay, where it is pricey.
Here are two perfectly nice bikes from the 60's that might easily pass for 1950's bicycles at first glance.
1965 Carlton Catalina
