At retail, no. On eBay, and other onliine sellers, probably, especially those who sell left over old stock. You just have to make sure it's a legit copy. Windows 8.1 is not a whole lot different from 7 once you add a third party menu program to it. It's pretty much the same under the hood. If your machine runs Vista it'll most likely run 7 or 8 with no problem. I even installed Win 7 on an XP machine and it runs good.
I have a product key available for a full version, not upgrade, of Win 8.1 that I bought for our church and never used. You can have it for what it cost us, $40. As has been mentioned, you cannot move up from Win Vista to 10. To go to 10 you must buy a copy of 7 or 8, or buy 10 outright.
As far as Bob's comment about it being spyware, that is true in that MS probably is collecting data and selling it. That's one reason why they want you to do everything in the cloud. But, and this is a big but, I'm not sure they hold a candle compared to what Google is collecting and selling. It's the nature of the Internet today. Hell, even though he's not going to confirm it one way or another I wouldn't be the least bit surprised if Lamont is collecting revenue from data mining that takes place on SL. SL shows up in Google searches so who knows how extensive the data mining is that Google does, with or without his acquiescence.