I think you are looking in the wrong places for the capacitors.
Antique radio parts, axial capacitors, electrolytics to 600V etc. here:
http://www.justradios.com/Aluminium electrolytic capacitors are fine for those parts, if you change them to EIA standard values it is much easier to find. No harm in rounding up.
But 450VDC is the max. electrolytic (common) rating so the 500V part is a bit of a challenge.
I would use a polypropylene film cap, Digikey has several 8uF 700V from Epcos or Kemet.
Replace all the electrolytics.
C15 -> 4.7uF 450V
C16 -> 22uF 450V
C17 8uF 500V same
C12 16uF 50V (28139) -> 22uF 50V
C10 25uF 25V (28141) -> 22 or 33uF 25V
These parts will be much smaller than the originals, so the leads are short and you must add teflon tubing and some wire.
I use a dab of silicone caulk to glue the film cap upside down, and tack the electrolytic caps side to the chassis if they are too loose.
I have a 503 or 700 amplifier in my junk box.
Very hard to find 6SC7 that are not microphonic for the input tube.
You can test the amp with a temporary power resistor (4.7k 25W) in place of the electrodynamic loudspeaker field coil, with ordinary speaker connected to the output.
edit: looked at Model 24-A schematic