Questions like Where to buy Open Source Computers & Which Open Source System to use has been there for ages, and the answer depends on what do you want to do with your open source computer.
Android is the most popular mobile Open Source operating system. It has got everything you may need 'on the Go'. Android is a "Jack of all trades, Master of None".
Chromebook, designed for average, everyday tasks, requires a constant Internet connection. As a cloud computer, Chromebook necessitates the use of hosting services.
Installing Ubuntu on an old computer is an excellent alternative, especially if you don't have 24/7 good internet connection or don't like your data in the cloud.
Gaming / GraphicsEdit
Gaming & Graphics Software requires high Graphics Processing capabilities, & the addon Graphics card(s) make these computers are very expensive.
Best would be to install Ubuntu or one of it's derivatives, like Mint, onto a Lenovo Ideapad Y or K series, or HP Phoenix series computer. If you don't want to do that, there are options available, but only in US and Europe. International shipping costs premium prices, as computers need delicate handling.
Valve is launching 'Steam Machines' with Steam OS sometime in the future, to fulfill this gap in the Open Source computing lineup.
If you want a full open source computer, where both Hardware & software are Open Source, you need to build it yourself.