I've decided to get my shopper wheels rebuilt with 451 rims rather than scrape around for 500A tyres. It wont be 100% original any more, but I'll have a bit of choice, instead of relying on a single supplier and tyre. 28mm Duranos should be a bit quicker too.
There's these 35mm whitewalls from SJS: http://www.sjscycles.co.uk/schwalbe-hs166-rigid-road-tyre-black-white-500-x-35a-%2837-440%29-pre-2011-prod12883/
I've decided to get my shopper wheels rebuilt with 451 rims rather than scrape around for 500A tyres. It wont be 100% original any more, but I'll have a bit of choice, instead of relying on a single supplier and tyre. 28mm Duranos should be a bit quicker too.