For film caps, it's probably best to buy all 600V. For electrolytics, get 450V. This is overkill for an AC/DC set, but it's easier to just buy caps with sufficient voltage rating for virtually any set you might run across.

Most run-of-the-mill places, like RadioShack and internet surplus pedlers, typically stock caps for use with solid-state ckts, which have too low a voltage rating for tube-type ckts.