@annabel unless you live in North London I wouldn't have a clue. @hardys might have them. Clockhouse Nursery have a few left. Other than that try the 'online' crews or ring and ask your local plant retailers.
@annabel it is a macro shot (very close up) you only see the superb detail when you are close up. They look like pinky-red heads, on tall growth. So I'd say middle/back of damp border or far side of a pond / stream, would be good locations.
Sorry to say not one we do .. I don't think we have grown any Asclepias in the last 15 years .. You could check out the RHS plantfinder on their website for a possible supplier https://www.rhs.org.uk/Plants/Search-Results?form-mode=true&context=l%3Den%26q%3DAsclepias%26sl%3DplantForm&query=Asclepias