I was also looking for a solution for active probe and eventually I found three options:
1. Do it yourself with a nylon braid (simple but a bit less flexible than I wanted - with micro coax cable
https://www.aliexpress.us/item/2251832707700559.html and wires inside). Using 36AWG teflon wires makes it thin overall.
2. Do it yourself, but with thin wall silicon tubing as jacket (same coax cable, tubing with 0.5mm wall, 4mm id). I saw that there are heat shrink silicone tubing available now, but I did not try how flexible they are after shrinking.
3. USB4 "coaxial" cable (it has 4 micro coaxial connectors and many wires, suitable for differential active probes). There are even "silicone" cables (
https://www.aliexpress.us/item/3256805198860016.html) which, if you order as a single piece, will come at any length. Or you can order longest cable (not really good for USB4, but you can cut it in pieces).
As far as I know, LeCroy and Tektronix were ordering their cables directly. I used to have Tek cable but it had only two conductors in addition to coax cable.
In all these cases, the miniature teflon coax cables are suitable up to few GHz. The USB4 cables have more roll-off at >2-3Ghz. And high frequency active probes usually need 4+ wires for DC bias and controls.