I need some fairly specific capacitors types, like polyester or silva mica.
 

Normally this wouldn't be a problem, but I have some PCBs that dictate that I use axial capacitors in certain positions calling for a low value, the values being:

270pF (220 should do)
340pF (330 should do)
500pF (470 or 510 should do)
1000pF
4700pF

The 4700pF and 500pF need a voltage rating of 400v or more, whereas the rest can be rated at 125v or more.

I'm having great difficulty finding suitable parts that aren't ceramic or insanely expensive (£1 per cap?)

Does anyone know where/if you can get these types at a reasonable price? NOS should be fine if I can buy a fairly large quantity.

I'm also looking for 330pf/500V and 100pf/500V 1% silver mica caps for a phono preamp. Don't care if they're expensive - I only need four.

Check fleabay. I got several batches of former Soviet Union Silver Mica caps at reasonable cost.

Like these 4700pF at 500V:

4700pF/500v./10% SGM-4 Silver Mica CAPACITORS (6pcx.) | eBay

No affiliation, blah, blah, blah....

if you're looking at NP0/C0G ceramic then they are superior on all measures to polyester, and if dielectric absorption is taken as a "audio quality" indicator they overlap with Silver Mica depending on manufacturer

polyproplylene is common and good, polystyrene would be most people's choice for RIAA filter

I have no audiophool neurosis with using ceramic as long as they don't introduce measurable distortion

As long as they're axial and fit, I'm not too bothered :P

Ive successfully used ceramic 4.7 uf coupler caps instead of grainy sounding (from the distributor that way) tantalum electrolytic caps. The caveat is I had a 1.6 V signal, and I used 50V caps. They are non-linear, but less so over only 1/25th of the curve.
That said, mica caps are available, I think from tubesandmore.com AZ, although it might have been triodeelectronics.com IL. Antique radio parts houses specialize in these. There is another radio parts house in Canada, I haven't bought from them so I don't remember the url.

Silver mica capacitors/Dipped silver mica

:)

Avoid X7R and Z5U, they are microphonic due to the use of type 2 High-K dielectrics which are piezoelectric.

Why don't people use polystyrene for coupling?

Maybe because of their somewhat higher inductance and heat sensitivity? Not sure though...

Thanks for the link. :)