lysamachia yellow loosestrife

Go to the RHS website on www.rhs.org.uk and find the 'Plantfinder'. This will tell you which nurseries in Wales have Lysimachia punctata (the yellow one) - I found three - but the white one is listed as 'widely available' - latin name Lysimachia clethroides.

That means that many nurseries stock it. If you still have trouble after ringing around, google it and you'll get a list of mail order nurseries. If that doesn't help, send me a PM and I'll give you details of a nursery local to me that has it.