Rhino's CD box set of every Allan Sherman album is now out of print, but you can now buy each album individually on CD here. Peter & The Commissar can be purchased on CD here.
All of Allan Sherman's albums are also available on iTunes.

Mark Cohen (he wrote the liner notes in the My Son The Box box set) is currently working on a biography of Allan Sherman. He wants to speak with anyone who knew Sherman (or has bootleg recordings of Sherman concerts, private performances, or letters from Sherman). You can contact him on his website here

Download My Son The Folk Singer Booklet (pdf format)
Not available anywhere else on the web (as far as I know)
Be patient, the file is large it can take a while to download