I'm not sure the main ones do any more, i used to be able to get them at sainsbury's & asda but i've not seen them for a long time now. I've done a search on my supermarket and nothing has popped up yet
Best from Booths if north-westernEngland....even better Penrith, Ullswater Road or the Westmorland stop on the M6, the farm shop, for Cranstons....they used to sell it by the foot. Dales in Kirkby Lonsdale is a good alternative....
Waitrose list them - limited stockists though. None of the big chains stock genuine Cumberland Sausage.If you are in Cumbria, your local butcher is the place to go. There's a Cumberland Sausage app coming soon http://thecumberlandsausageclub.org.uk/ , that will give you locations. Mail order - as someone else suggested, Cranstons currently or Ocado.