Some of the suppliers I've used in the past (mostly American!)
Jas Townsend and Son Warning: This site is addictive - you will want to buy EVERYTHING. They're very good with their delivery times and stuff, too.
G Gedney Godwin Not quite as extensive a collection as Jas Townsend, but still interesting. I've not actually bought anything from them, so I don't know what they're like with deliveries, problems etc.
Patterns and fabrics suppliers:
Kannick's Korner Easy-to-follow patterns for a wide variety of things
Patterns of Time If they haven't got it... it probably isn't worth having!
Wm Booth, Draper A decent selection of period fabrics
Period Fabric Again, a decent selection, though the website isn't completely up to date.
Ticking If you're like me and need ticking, this looks like a good place to get it - they have it in a variety of different colours, too. A little pricey, though.