BNC connectors are highly shielded, bayonet connectors with constant 50 or 75 ohm impedance for high frequency RF use.
We used to call them "British Naval Connectors", but that is not actually true, the design name is actually "Bayonet Neill–Concelman".
So, radio shops have them. Commercial, Ham, etc.
The security industry took a liking to them for video, and CCTV vendors also have them.
Old computer dudes will remember them as early Ethernet Base2 connectors.
They are very easy to find new or surplus, and the Mil-Spec surplus variety are excellent.
Keep to 75 or 50 ohm and don't mix them, although they can be mated, you'll get the best fit with the match.
Be sure the cable-end diameter makes sense, as that is the biggest variation in design.
Great, durable connector, much better than phone plugs or phono plugs...
Amphenol and Neutrick are my go-to brands.