I bought two last year from XKs Unlimited.
A word of advice. This applies to XK120 and XK140.
I recently bought a reproduction advance unit from one of the regulars. This is the version with two threaded studs on the vacuum chamber to hold it on to the distributor body, used on Mark V, XK120, Mark VII and XK140. It came in a Lucas box but does not have the Lucas name on it.
To remove the vacuum unit, remove the two nuts, then turn the micrometer knob all the way, at which point the studs should be clear of the body and you can turn it …