I think you would have problems even finding a Windows 7 license
Yeah that was going to be my next question because I don't want to use pirated or "fishy" OS software. I'm not even sure if it's "safe" to purchase on ebay. Was hoping that there was some credible sources that still offered it. Probably not though?
Guess if I do one then it's Windows 10.
At the very least eBay is safe-ish. hahah - not much help, am I? But I've purchased MS_DOS 6.22 multiple times, Win98SE, and Win7 at least twice on eBay, all with a Certificate of Authenticity, license label, etc. Haven't had a problem with any of them, either installing or registering them with MS (Win7, anyway - the other two don't phone home).
Whether you go that route or Ryan333's suggestion of Amazon or Newegg, make sure you get a full copy and not a Student or Upgrade version. Those look tempting because of price, but I'll suggest sticking with the full install retail version since that seems to be the easiest way to avoid hassles during installation. I also suggest you make sure it comes with Service Pack 2, else you'll spend forever and a day getting it updated after the initial installation. Even with SP2 it'll still take forever to get all of the updates, but you'll save that day at the end. ; )