There are a number of Internet web sites (Google search) with additional information on rebuilding HP-23 power supplies.
Here is schematic and operational description:http://www.theheathkitshop.com/hp23.html
Repalcement electrolytic capacitors for the HP-23 are available from a number of sources.
The best prices for electrolytic capacitors are found at Mouser Electronics. www.mouser.com
You can order by phone or Internet - very helpful staff.
You can also look at The HeathKit Shop and Mike Bryce's repalcment PC baord design (this was featured in a past QST article)http://www.theheathkitshop.com/hp23r.html