There’s nothing illegal about buying an OEM key, so long as it’s an official one. There are plenty of legitimate sites online that deal in this kind of software, Amazon has a number of sellers offering OEM keys, as does eBay, and more specialist sites like the aforementioned getmskeys.com are an option.
So long as you’re happy to take on the responsibility of being your own technical support, then an OEM version can save a lot of money while offering an identical experience.