I bought mine from Schumacher a couple of years ago when they still had them in stock. It looks like they no longer support much but I had heard that US CarTool had picked up the line of mounts and would be making them.
However, both Mancini Racing and Classic Industries show them as "coming soon" MA4901 - 1967-72 A-Body S6 to 5.7/6.1 Hemi Conversion Kit Engine Conversion and Replacement Kit