There is no specific source for old transformers. You only have a few choices to track one down. The usual online sources (ebay, craigslist), hitting used parts dealers (google and you will find some, or hit the vintage guitar show circuit, you might get lucky), or take the part number in hand and hit every surplus store in the nation and hope you find one.
Best bet is probably buying a new one though. Mercury makes great stuff. So does Heyboer and a number of other makers. Plenty of choices for those transformers.