I would bet good money that your Sun wheel hubs were made by the British Hub Company. This firm seemed to provide the vast majority of pressed steel hubs (including the brakes) for all makes of British lightweights at the period of your Sun. If that's the case you should keep a look out for almost any 5" wheel hub, Fanny Bee, James, etc, etc, of the period and check the brake shoes out. In the early 50's British Hub were starting to move over to cast aluminium brake shoes rather than the previous pressed steel so you can find either type. Unfortunately they are not quite interchangeable without a little fiddle with the fixed fulcrum post.
British Hub fiddled about with the non essentials to suit various makes but the basics remain the same regardless. Luckily the brake shoes were one of the basics.
As far as you finding Sun plunger components I can't help.
Best of luck.
