Can find every size but, mainly using charliebikemonger.
Can you find one?

Yes they do, I’ve got a white industries 17t freewheel.

am sure I’d seen news that one was now available

Surly do them, also try googling “endless cogs” or try chrisking.com

SSP

Do 17 teeth slot on sprockets exist?

Yes they do, I’ve got a white industries 17t freewheel.

????

That’s a bit like;

‘Do yellow cups exist?’

‘Yes, I’ve got a purple one’.

http://www.on-one-shop.co.uk/acatalog/Sprockets.html

groove armada from On-One in 17T and in stock. Not many other freehub sprockets in 17 around. On a stell freehub you could use a sprocket taken from a cassette.

Many thanks, used old cassette to find 17tooth, hacksaw to cut pins and fell apart. Works like a dream.
Will try out before I commit to On-one version.