The nature of Coaxial cables is that they are all stiff. The intention is to maintain the spacing between the core and the screen so the permeability stays the the same. This means the geometry of a cable should stay the same along it's entire length.
Those that flex or have been bent are now no longer perfect. For most applications this perfection might be desirable but is not required.
In a production test environment the length of the cable (connecting instruments) are usually determined by where and how the equipment will be tested.
So the advice is either lay the equipment (to be connected) out with plenty of clear space to allow everything to "float" in to position that doesn't stress anything to much or make cables the right length for the task.
The OP's task of finding flexible cable will be as hard as user6972 said and would be a waste of time unless absolutely necessary.