John, are you sure it's an HP-23, and not an IP-23? Is this one of
their high voltage power supplies or a low voltage model.

Whatever, your most likely bet is going to be eBay. Here's two of the
Heathkit power supplies being currently auctioned.

http://cgi.ebay.com/Heathkit-Regulat...QQcmdZViewItem

http://cgi.ebay.com/HEATHKIT-REGULAT...QQcmdZViewItem

http://cgi.ebay.com/HEATHKIT-REGULAT...QQcmdZViewItem

Good luck. Harry C.