where to buy N52 neodymium?

There are various specialist online suppliers of NdFeB magnets on the web, first4magnets, supermagnete.de are two that spring to my mind from memory. Do not trust eBay/Amazon if you have particular requirements like this.

You actually want to purchase bulk product? Unmagnetized or magnetized. I'd talk to one of the specialist suppliers like those two (I'm sure thare are more). You know NdFeB is sintered and nickel plated to prevent rusting (the bare ceramic rusts like crazy).

Be aware there are very stringent rules for air-mailing large magnets, so you will probably be limited to surface mail for such products.