The Performance Center used to have them back when there was a real Performance Center. They had a deep one and a shallow one. My Tactical .40 came with a shallow one and I ordered one from S&W for a custom 5" 5906 I put together:
The bottom of the frame has to be milled to fit the aluminum mag well. The one I ordered was the deeper style so I filed it down to the size I wanted.
These are probably long gone, but if you find a parts diagram from a PC gun with the mag well you at least will have a parts number to ask S&W if they have any left for sale. That's what I did, but it was like 10 years ago.