1-800-4 Champagne (US)
75cl.com
Amivin
Berry Bros. & Rudd
BottleNet
Champagne Cellars
Direct Cellars
Drinks Direct
Fine & Rare Wines
Just Champagne (UK)
Laytons (UK)
Majestic (UK)
Oddbins (UK)
Shopwine
World Marechal (FR) A good selection of Champagnes at bargain prices.
Wine & Co
Wine Cellar

Wine Searcher.com is a useful database of various wine merchants' stocks. Simply enter the Champagne and vintage you are looking for e.g. "Bollinger" & "1990", and a list of prices and availability flashes up:

Wine Name:
Vintage:

Customised Labels

Customisers.co.uk