The only place I know of is gaussboys.com and they are, IMO, extremely reasonable for the quality of magnet that they are. I've purchased 1000s of magnets from various sellers from around the world, and the quality of their magnets is without a doubt the best I've seen. So I think you really get what you pay for, especially with them. Plus, the magnet will be up to spec.
There's been a trend lately by MANY sellers of magnets (mostly on ebay) to misrepresent what they're selling as higher grade than it actually is. I had at least eight different comments from people just yesterday on one of my videos tell me that they've used N50s in cubes such as the Air, Valk, and GTS2, and then tell me that these magnets actually felt subtle in these cubes! That was just yesterday, but I've been hearing this over and over almost daily now. These magnets are probably ok to use but should be approached as N35.
Anyway I just bring this up that most often you'll be better off dealing with someone whose sole business is magnets and someone who's easy to communicate with. Ultimately the few dollars more that you spend assures you that you'll be getting the right magnet, the plating will be durable and high quality, and you'll receive them in a couple of days. In the long run, it's the better buy, I think. But I do certainly understand how tempting some of those lower prices are!