I didn't see anything in analog888's listings, nor any offer for custom values. DIY Gene also sells R-cores on ebay:
50W R Core Transformer,115V//230V, for Preamp,DAC/R26-x - eBay (item 260307179697 end time Nov-27-08 05:34:16 PST)
R26-42 0-50-55X2
would work, although it is 50VA, which I guess would be more than enough. I had heard the same thing as you were telling me though that going up a bit on the xfrmr rating could sound better. He has some 100VA, but nothing with the right secondaries. Plus, this was the guy that was doing the clone amps (and probably still is), so I'm not sure I want to deal with him.