Question of a source for two specific parts of the clutch pressure plate assembly pops up fairly often. Specifically items 2 and 3 in this illustration can often be the difference between an inexpensive repair or far more expensive replacement of the MGA pressure plate. The trick to finding parts like this is to get the original BMC part number from the MGA Service Parts List, and then do an internet search.

The items of interest are:
(2) 7H3001 Retainer (spring)
(3) 7H3213 Release plate

As these clutch parts are shared by some other vehicles, I have found these parts at a Nash Metropolitan parts supplier, Kip Motor Company in Dallas, Texas, USA
See the clutch catalog page www.kipmotor.com/Catalogs/MET-clutch.pdf   (0.4-MB file)