As far as I can make out there are no UK growers of dahlia Yvonne.
However, if you really want to get some brownie points and a big hug, this plant grows wild in New York Central Park !
There are many USA growers and suppliers which can ship over to here but the plant must be declared.
Southportsmith

You can buy Dahlia Yvonne on line from Withypitts Dahlias - Specialist Dahlia nursery in West Sussex.
www.withypitts-dahlias.co.uk/home.html hope you get your brownie points!

Literally just round the corner from me!

Plant Finder also lists Abacus Nurseries in Neath as a supplier (who offer mail order)

Thank you everyone for your assistance and so swiftly too!