You need to be looking in the mower engine bone yards for a rebuild-able one.
It appears that the great financial investors that current pulls Briggs strings have decided that the carburetor is no longer available from them since it a low volume item.
This is the way the industry is currently tending with OEM no longer supplying parts for many of the older engines. Even some of the new engines now are having parts availability issues. Just recently I had a less than two year Briggs engine that I could not even get an ignition coil for. Then there companies like Lincon that simply don't provide any parts for their engines.