K&J Magnetics is probably the most popular single US supplier. I've ordered from them, myself, and was quite happy with the price and service. That was some time ago though, and I've heard grumblings here and there that while the service is still good, their edge in value has dropped, somewhat.
Seems that most people are going to Ebay, nowadays, as better deals can be found on smaller packs of magnets (just keep an eye on shipping costs - sometimes combined orders are the only way to avoiding paying
more, once all is said and done). Unless you find something useful in their clearance section (which is worth checking - most sizes/shapes are practically worthless, to us, but there are occasional gems in there), you only start to see serious savings from large suppliers like K&J when you order in bulk.