WEE Technology Company Limited - WEE are Capacitors Specialist

I highly recommended WEET copper foil caps. I prefer them to Duelund and have tried both. Far superior to Mundorf in every way, but most especially in the mids and micro details. The WEET copper foil cap is very linear with no part of the sound spectrum highlighted.  I find the top of the line Mundorfs a tad boosted in the highs. The Duelunds a tad bumped in the upper mids. The WEET are so smooooth!

You might want to try DIYAudio, but in the "affordable" boutique crossovers my favorites so far:
Clarity MR (now CMR)
Clarity ESA
Mundorf MKP
Mundorf Supremes
WEET WMW copper foil caps

I also like Audyn Truecopper corss to WEET WMW copper foil caps (Bypass values only)
There are two main types of quality caps: metallized, and film-and-foil. The former are cheaper, the latter better: by better, I mean smoother, more refined, more detailed.
Dielectric thickness doesn't matter very much, but metal thickness does. Just about anything thick enough to be wound up on a spool is thick enough for sonics, but a deposited conductor is a different matter.
Construction is also important. The larger the capacitance, the more the cap can suffer from inductance. MIT Multicap have a connection scheme that minimizes this. Duelund has a flat pack topology which is just about optimal.Polypropylene tends to sound pretty good, but compared to styrene, is dull. PTFE (teflon) sounds a little bright compared to styrene. Styrene is, to my mind, the most neutral of the conventional dielectrics. These are the films.For metal, the best is one which connects easily (usually solders easily). Those are silver, copper, and tin. These are the foils.