You might try a video truck rental company like All Mobile Video in New York. They have a rental house also, and I know their trucks carry these so maybe you could rent a set.
Another place would be to try local jib owners. It used to be pretty standard equipment before internal jib cabling became more prevalent. I think there's a page on the Stanton (Jimmy Jib) website where you can look up local owners.
Check around and see if you can find the chassis mount versions. They aren't as pretty as the triax bullets but they will still work the same. I needed a few of the adapters a while ago and of course they weren't available. I found a supplier that stocked the bulkhead-type adapters and they worked perfectly. They are designed to be mounted to a cutout panel on a rack to hold the adapter in place with the backside having the triax mount and the front side having the BNC mount. Actually cheaper than the bullets as well.
The name is James, From Bristol, UK. Currently a media student, doing what I've always wanted to do. Which is playing with camera's filming music videos etc!
Anyway. On to the subject.
I've been looking around for these Triax to BNC connectors but cant actually get anywhere with them.
Being a student, I dont really have a lot of money. So I thought I would look for a triax connector, which will hopefully get my Sony CA-55p triax output to a bnc.
But I thought I would try the option of taking the triax connector out and soldering in a panel mount bnc. All done, but does not seem to work. I have no idea why.
I thought someone here might be able to help out :)