Question: Why are you looking for ceramic discs at all?
Times have changed! If you're used to the olden days (I kind of get that impression, sorry if that's incorrect), ceramics were used all over, but they're very expensive now -- it takes so much more material to make them than other kinds. And the materials have improved to the point where the same values can be had in microscopic sizes, 1000s of times smaller than vintage discs.
If you still need through-hole, moderately to high voltage rated parts, film capacitors are the way to go. Polyester is good for general purpose (timing, signals, and low ripple filtering), while polypropylene is excellent for higher stability, and high ripple filtering applications.
This includes line filtering, which requires an X1 rating for line-to-line applications. Y1/X2 ratings are for line-to-ground, which can be had in film types, but ceramics are usually cheaper and preferred there.
For very high frequencies (such as you'd see in RF amplifiers), MLCCs still offer good performance, as long as you get C0G type dielectric, and a suitable size (e.g., a 1nF 630V part will be in a 1206 SMT package, or similarly sized dipped and leaded THT part). Discs are still reasonable for small values (maybe 100-200pF and below), but larger values (aside from Y1 rated types) just aren't economical anymore.
By the way, for RF purposes, avoid type 2 dielectrics. Lossy, even when rated for much higher voltages than your need. Get C0G (sometimes labeled NPO), or something nearby (N750 or other tempco -- if you don't mind, or actually need, a tempco of course!), or SL at worst, I think.
Tim