Britman,
First,
I wonder how you happened to discover the forum. Google? We're always interested in knowing how a new member came to own their A7/A10 and, of course, photos are great. Maybe you are so new to British bikes or computers that you are not familiar with the well-known online parts distributors. The first three that I just checked all appear to have the pistons you're looking for:
Britishonly.com
Degroot-bsa.nl
vintage-motorcycle.com
Let us know if you need more choices.
Having said this, I now need to check to see if we have a topic where all known online distributors are listed.
If you happen to have a ton of questions, no problem, folks here are happy to help. If you happen to have a ton of help to give, folks here (like myself), appreciate it.
Richard L.