I've just seen your posting and wondered if anyone granted your request. 'Checkers' is commercially available, but back in 2013 our small local park had a dark purple-flowered Aster growing in a large clump. I made a request to the Council and an estate manager arranged to meet me there.
He dug up a couple of roots, and they've been growing in out garden ever since. He couldn't actually tell me what variety it was, but guessed at 'Chequers'.

Thanks for your reply - I have now tracked down a supplier, and it is growing happily in my garden

That's good to know!