As for where to buy the parts, there are several really good sources. I tend to use Michael Percy Audio and Partsconnexion.
When I got serious, that is when I retired, I built a break-out box to test capacitors for speaker applications. My experiments (two alternative forced choice, single blind paradigm) showed that the most important variable is technology: film and foil is best alternative, then metallized, then other. Second most important is materials: among the metal foils silver is best, but prohibitive; copper almost as good; tin is cost-effective; aluminum can be problematic because of the connection between the wire lead and the metal foil. Among the insulating films, styrene is most neutral, teflon somewhat bright, polypropylene somewhat muted. Third most important is brand.
The best manufacturers are very open about the technology and the materials they use - they brag about it, and for good reason. You can profit from their openness.