I have the W3NQN BPFs built by Bob 5B4AGN. He uses custom capacitors ordered and manufactured in small batches. I don't know the technical details but there's probably a good reason for Bob to resort to using the expensive and custom capacitors. His W3NQN filters are one of the best.
Well, there you have it. Unobtanium. You'll just have to buy the real filters and stop fantasizing that it's possible to actually build something on your own using alternative methods, instead pf buying in to boutique stuff so you can be in the club.