I have seen a few people reference Lee Valley as a source for rare earth magnets. I thought I'd suggest a couple other places I've come across.
1) ebay - If you want a lot of a particular size, I have found several people there selling large lots of magnets for pretty good prices.
2)
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- I have ordered from here. Their prices seem a little lower than Lee Valley. My order came very quickly, and they have a very good selection.
3)
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- This place has some odd magnets. If you need something really powerful, this may be a good place to go. They deal a lot in surplus magnets and have a small selection, but they're in unusual sizes, some VERY large.