You don't buy Windows. That's free. Can be downloaded from many places, but the most secure site is MS itself. What costs is the activation key. You can go through any major retailer such as Amazon or Newegg and get the same key as from MS. A windows Pro key will cost more than Windows Home, so if you want to change from Pro to Home you can, just DL a copy of Home, install that instead of Pro and get they key for the new version.
Any major retailer is going to he equitably priced to MS, within a few $, unless you catch a sale.