Floppies turn up regularly on eBay of you want small quantities, and you can get them out of the states new still from Athana:

http://www.athana.com/html/diskette.html

Disks: I've generally just picked up lots of blank discs when I've seen them on eBay etc.

Or as Mark says, you could order them in from the US e.g. Athana. This is the quote I had from them a couple of years ago: "500 5 1/4" diskettes in envelopes for .52 U.S.= $260.00 plus $117.00 for shipping to the U.K. for a total of $377.00 U.S."