I recently saw one on EBAY . It had a 'Buy it now" price of around $1000 and the bidding started at $500 or so. It never made it above $600.
I thought I'd need it for my OMC Cobra -->Merc install but the Cobra bolt pattern/cutout is EXACTLY IDENTICAL to the Mercruiser Alpha (I need to drill 2 additional holes for the Bravo) Newer Volvos (sx?) should be the same too I would think.
Try Outdrives.com or Mercstuff.com. They may have one of those plates.