Obviously, the easiest thing to do would be to watch any Warner Bros. movie from the period, as they only did contemporary films back then, very little period costume drama and so the clothes would be pretty authentic. What about The Grapes of Wrath as well, or Dead End?
The Salvation Army will probably be picked clean by costume people (and nowadays, they sort and sell the good stuff to the movies.
Why not contact a nearby semi or professional theater company and see if they will rent you stuff from their costume house? I used to do this and many companies would for the extra income.
http://www.broadwayworld.com/board/readmessage.cfm?thread=972787#3631451
http://www.broadwayworld.com/board/readmessage.cfm?thread=963561#3533883
http://www.broadwayworld.com/board/readmessage.cfm?thread=955158#3440952
http://www.broadwayworld.com/board/readmessage.cfm?thread=954269#3427915
http://www.broadwayworld.com/board/readmessage.cfm?thread=955012#3441622
http://www.broadwayworld.com/board/readmessage.cfm?thread=954344#3428699