Jaran,
I think you will find this is a common part available from any of the larger British bike parts houses. Here in the U.S. a company called "Domiracer" recently bought out the large inventory of new BSA parts of another big online parts supplier, "British Only." Also, there is "Britcycle" and others in the U.S. In the UK, "SRM," "Draganfly," "Leigh Classics," and the list goes on. I looked up your part in my parts book and I believe I identified the seal you are looking for. Here is the link to the Domiracer part:
https://www.domiracer.com/comersus/store/comersus_searchItem.asp Please check to be sure this is the correct item. If you do not own a parts manual, you'd better buy one of those first.
Richard