cjacek beat me to it... I hooked up with a 38 locally (pittsburgh area) through craigslist, and I got a good deal on it. I placed a "wanted" ad on craigslist and got a reply fairly quickly. In the past two years, this was the second one that I came across in the are (first was in a local music shop), and I had not really been looking that hard.
My guess is that it will harder to find a 58 or 48, as they just don't seem to be as common. The 38s are not really that hard to find, and Green Bay is a large metropolitan area, I think you should be able to find something locally if you are patient.
Correct me if I'm wrong, but I believe many parts of the 38/48/58 decks are interchangeable. I don't think it will be any harder to locate parts for a 48 than a 38.
What are your plans for the R2R deck? Are you running a commercial studio or is this just home recording? Do you plan to use it as front end for a DAW or an all analog set up?