I'd love to share, but I don't have any Mopheads. All mine are lacecaps and are just beginning.
Mike

Mine are likewise all lacecaps and are not quite out in full bloom. There are several large hydrangeas in my neighborhood that are a-bloomin' their giant mop heads off, but every one of them is blue. If you don't get a favorable response here, you might want to do a bit of driving or walking around in close-by neighborhoods. Chances are, if you find a gorgeous white mophead (and compliment its owner profusely) you will find a generous gardener willing to share some of the beauty.

Good luck in your hunt!

Rain

Have you been able to track down what you need? It would be leaving it a lot closer to the 'last minute' than I might be comfortable with for planning wedding flowers, but the Seattle Wholesale Growers Market is open to the public a few hours each Friday, and they might just have the white hydrangeas....they do offer cut hydrangeas in summer.

Here is a link that might be useful: Seattle Wholesale Market

Not white, but as long as you're in a pinch and don't mind blue & occasionally dappled purple, I have a massive one that you're welcome to raid. :)