These boxed copies of Windows 7 are still available, even though Microsoft is no longer selling them. You can find them on any site that sells computer stuff, from Amazon to Newegg. For example, perform a search for Windows 7 on Amazon.com and you’ll see you can buy a boxed copy of Windows 7 Home Premium for $89. Note that this is an OEM edition, so you’re only able to install it on one PC ever — that’s what the license agreement says, at least.
This isn’t a terrible deal, but you may want to reconsider spending the $89 and try using Windows 8.1 to see if you like it. You should also try to ensure that your PC’s hardware offers full support for Windows 7 before purchasing a copy.