Between the time Third Stage and Walk On were released, Brad Delp had left BOSTON to join Barry Goudreau in a new band Called RTZ. They released their first album, Return to Zero in 1991 on the Giant Records label. For many years, that album was out of print and could only be (legally) obtained by purchasing a used copy, or if you were lucky, some obscure music store had an unsold copy somewhere.
Recently, this album has been made available as a digital download. It is available on iTunes and at Amazon.com. The page at Amazon shows the release date as April 02, 2013.
If you're not familiar with RTZ, you can check out the RTZ section of this site.