I recently acquired SpecAn for 26.5GHz, HP8593E with N fitting for input port.
What I didn't realize was, everything will become so hard to obtain; even silly things. Even high quality DC cutoff, cable harness, connector adapters, etc, etc, etc, will end at 18GHz. Moving to SMA isn't that much better. 3.5mm is super high cost, fragile, and again, not as available.
I have lower grade stuff connected because I don't need that kind of frequency response very often. But when I do, I'm having have to consider if decline in level, non linearity in trace, etc, is due to connection or DUT.
I have a very good surplus source locally, so once a week, I'm there looking for this stuff. Occasionally, I find them. Only thing new I see that goes beyond 18GHz is usually VNA related ultra stable cable for thousands. I cannot afford that.
What do you do???