I ran into this problem a while back. They are somewhat rare but not impossible. I never found a reliable source for production but I was just making lab stuff and found a couple solutions.
One place they are used in twisted pair coax balun type transformers used for CCTV systems. These are cheap enough that you can buy them from china and take the bits you need. See photo.
I also made a few adapters from BNC T's and L's and a few minutes with a dremel tool. Don't have a great picture but attached what I could.
Below is a quick picture of both. Top is label from bag that Balun, left, came in- MC Electronics is a low tech electronic distributor in the US but these can be found elsewhere. Right is a little project that has a 20 Mhz crystal and an NE-602 mixer that was a receive converter to get 162 Mhz Weather on a 2M radio. I've used it for years and its pretty beat up. 9v battery on clips and BNC directly into radio- carved the adapter with dremel wheel to expose center core and outer cylinder. Soldered up pretty well