"Have you tried to buy a polystryene capacitor lately?? Sorry, no longer made."

Soundhound, this place has them:

http://www.welbornelabs.com/rel.htm

In the interfaces of my Acoustat 2+2s and 1+1s, I have upgraded the capacitors on the high frequency low voltage side to include a 1.5uF polystyrene cap, a 10uF polypropylene film and foil and a 47uF metalized polypropylene. Gone is any electrolytic.

At my work, I sometimes come across small value polystyrene capacitors in the range of a few hundred pF to 4nF at 630v. They are usually in some old lab that is being cleaned up or out.

It is good that some of the old tubes are still available as "nos: new old stock" or are actually being made. When the semiconductor industry is sufficiently mature (i.e. something newer has came along and made it relatively irrelevant) the best of the last of the semiconductor devices may enjoy a popularity if enough people want them and are willing to pay for continued production of a then-to-be niche product.

My favorite tube of all time is the One-D-ten-T. Lots of them where I work.

Paul