The idea of a 'super massive white hole' is sometimes discussed regarding GR,SR and the destination of material 'lost' in a black hole. Is there any good theoretical or observational evidence for these SWH, or are they merely metaphysical speculations?
White holes are a theoretical construction of General Relativity.
Despite extensive searches, nothing has been found: not a supermassive white hole, not a stellar-sized white hole, nothing. Thus, the answer to your question is: on our current knowledge, they don't exist anywhere.
It is unclear how could they be formed. The geometry of the Kerr-solution (i.e. rotating black holes) suggests a white hole could exist on the "other side" of a black hole, but currently it is pure speculation.