You can certainly get this plant on line - Bluebell Nurseries currently have it in stock, and Burncoose lists it, not sure if they've got any available.

If you want somewhere closer to home, the RHS site lists Larch Cottage Nursery in Penrith - looking at their website it might be worth a trip out on a nice day.
I'm fairly certain that I saw this at Binny Plants (just off Jn3, M8, or Jn2, M9) a few weeks ago but it isn't listed on their website - worth giving them a call.