Where can i buy windows 7 in 2019

First of all check with the device manufacturer to see if they support 7 and supply drivers for Windows 7 for your exact model number.

If they don't forget about installing Windows 7.

Support for 7 ends in Jan, 2020 and if you can get it, it is expensive.

Microsoft no longer sells 7; try amazon.com, etc..

Warning:

You do not buy a Product Key by itself, as they are normally stolen/pirated, Volume Licenses which are not for private use, or MSDN keys which are not to be sold by the original subscription holder.

Make sure that you buy the complete sealed package (DVD, license)

https://www.microsoft.com/en-us/howtotell/Shop.aspx

Read Section: Beware of Product Keys sold separately.

It is NOT considered Piracy by Microsoft Staff who I have spoken to on many occasions about OEM System Builders Licenses

I don't consider it to be a wise investment as support for 7 ends in Jan, 2020.

A lot of people don't like 10.

My replies here are based on physically doing what I advise posters to do in my Computer business and seeing results of what others do in the real world.

You can ask a friend who is willing to give up his/her previously purchased retail copy of Windows 7 provided the copy is not currently installed, activated and running on another PC.

Besides the afore-mentioned, it is highly unlikely that you'll be able to purchase a Windows 7 Full Packaged Product (FPP). All that you can get are OEM System Builder copies of Windows 7 on craiglist, Amazon, eBay etc. And technically, you are not allowed to install it for personal use according to Microsoft Licensing Terms for OEM System Builders software. Detailed information can be accessed here. It is considered piracy because it's an authorized use of the software.

You're encouraged to stick with your Windows 10 to avoid all the unnecessary hassle and illegalities associated with obtaining Windows 7 at present.

In the absence of any official documentation from Microsoft to refute 

OEM Microsoft software clarifications, plus, “If it’s labeled OEM, you should stay away from that,” other than hearsay, you should disregard it in it's totality.