Here is another common place
CNC Routers, Routers for Wood, Routers for Plastic and CNC Plasma Cutter
Where is a good place or places to buy them only know of one just want to see if there is more options out there...
here the one i know of
Plasma Floating Head
Two more places for a floating Head Z axis:
Precision Plasma LLC
K2 USA CNC Router, Cabinet CNC Router, Small to Large CNC Router Machines, Desktop CNC router.
The "floating" part is just a slide and torch holder that will mount to your existing Z and uses the end of the torch as a probe to find the top-of-material. It has an added benefit of acting as a shock absorber in case something happens and the torch moves too far down. A secondary limit switch can be fitted to stop machine motion if the torch gets pushed to far up