Not sure why you are having trouble finding 600V replacement capacitors.... You will, of course, not find wax paper capacitors, but rather modern film types, either axial lead or radial lead, either one of which will work.
In addition to the already mentioned links to parts suppliers, justradios has a good offering of replacement capacitors including 600V.
http://www.justradios.com/capacitors.html
The entore Sprague Atom line of axial lead electrolytics is still available through Mouser and others.