These are much easier to find, and rarely requires parts with more than 100v rating. Placing two capacitors of double the desired capacity in series that have about a 300 volt rating will work. Both caps should have a 100k 3 watt resistor in series as shown below. Other likely causes of problems Open cathode resistors are a common cause of failure. This is usually a result of running the amp with bad tubes, or improper bias setting.
Tube sockets with splayed pins or extremely heavy oxidation are also common in old amps.
Cleaning may seem tempting, but replacement of the sockets is by far the best route in this case. Newer sockets will have tighter tolerances and better metallic materials for conductors. Some people replace internal wiring with newer, higher quality stuff. Commercially available wire from companies like KimberKable can yield significant audible improvements. If you need to replace sockets, take a good look at the wiring for signs of oxidation.