Many places sell them, but only a few people actually manufacture them, and the "source" is always the cheapest, if you can meet their minimum order needs.http://www.steward.com
(A real ferrite manufacturer.)
RF Parts (http://www.rfparts.com
) has pretty good deals on Mix 43 snap-on filters of various sizes, and the larger the quantity, the cheaper they get.
Lucky for me, I can buy them locally at Fry's Electronics (who does not mail-order, but has several retail stores in the west) at lower prices than the mail-order houses, so I just walk in and do so!