I have always had good service and quality spares for my BSA from C&D Autos in Birmingham and from Lightening Spares. They may be worth a try if you have not already done so.
Have a look at the Vintage Motorcycle Club's site/shop. You do not need to be a member to buy. I have got nice waterslide transfers on my 1929 BSA Sloper and some to put on a 1932 500 Sloper if I ever find enough parts to finish it.