[Electronic M]

Paper caps have DC working voltage. Film caps on most sites often have Both AC and DC working voltages*. It is the DC that matters. (IIRC on my links I added a 630VDC filter for the film and a 450V for the lytics so you won't see a DC voltage selection column unless you nix that criteria in those links). You can always go higher voltage on film and to a point lytics as well.

Paper and film caps are not polarized, but if you want a rough idea of the DC polarity on them there is a convention....The 'outside foil' band on the end of paper caps is traditionally connected to ground. The outside foil wraps around the cap shielding it (thus why you want to connect it to ground when trying to optimize performance). Somewhere on youtube I once saw a video on using a scope to find the outside foil of a cap.

*In tube amp circuits the DC rating is what matters since most paper caps have ~200-600VDC on them and maybe 5 volts AC signal (until you get to TVs and sweep circuits then it jumps sharply). If on your tube set there is a cap between the two terminals of the power cord or between 1 terminal and the chassis then AC rating becomes important. Many film caps at made at paper cap ratings are made as line to line or line to chassis RF noise suppression/safety caps thus the AC ratings being available.

The non-stocked stuff can be annoying, but what sucks even worse is stuff with a low stock...I was going to buy IIRC 10 .0047uF 6KV caps for some electrostatic sets I want to finish (+ a few extra for the future) and at checkout found only 3 were in stock the rest would be back order....So I dropped it to 3 and now I've got my fingers crossed that it will be enough (or at least the 2 in series I used in a roundy as an emergency sub for a 2KV part are the right value and that I can take them and can come up with something different for the roundy).