Hi Bart+4,
I would try Longstone Classic Tyres.
Search under 500x15 Dunlop Racing tyres and you will see a selection of tubes as well as the tyres.
Longstonetyres.co.uk
Good Luck.

Ian
2011 Plus 4
Wild Mogs Reims 2019, Goodwood 2018, Isle of Man 2017, Carried on to Caramulo" 2016