Officially, the only version of Windows that you can purchase standalone is Windows 10. Sales of Windows 7 ended on October 31st, 2013, and retail availability Windows 8.x ended on September 1st. Unofficially, you can find SKUs available on Amazon and elsewhere for both of these versions, and in several different editions.
PCs with Windows pre-installed follow their own calendar, though. Almost two years ago, Microsoft announced that Windows 7 PCs will be available until October 31st, 2014, with an extension for Windows 7 Professional that will be at least 12 months after… whenever they decide to announce the date. They announced the date a few days ago and, you guessed it, it's 12 months from then: October 31st, 2016. They also announced that PCs with Windows 8.1 pre-installed will have the same end of sales date.
So basically, you can only purchase Windows 10 now, and PCs will only have it pre-installed after October 31st, 2016… officially.