Where to find cheap PCB-mount BNC connectors?

As for most parts, if you order more than just a few pcs, LCSC is the cheapest source of these components (however, you need to consider the rather expensive shipping costs).

Here’s some BNC connectors:

• Right-angle connector 50Ω: dosinconn DOSIN-801-0050 0.361€ @100pcs
• Straight connector 75Ω: dosinconn DOSIN-801-0057 0.9118€ @100pcs

I could not find any straight 50Ω connector.

Note that I don’t use them in typical RF applications, so I don’t have any information on impedance matching or accuracy etc.