I buy all my Linux cloths from HelloTux: https://www.hellotux.com
Their statement on the about page wonderful!
We believe that free and open source software is better than proprietary. We also believe that we can change how people use technology. If you are the kind of person who likes freedom and fun, promote free software with us!
And they go on:
HELLOTUX is a family project of Gábor Kum, a Linux system administrator, a Linux user since 1999. His wife Maria and his children are Linux users too.
As far as they explain they only use free software to create the patterns for the shirts (and now also backpacks!).
They also support FOSS projects with every sell they make:
This is not all. From your purchase we support free software, usually $3-5 from each product sold. But not everybody can accept money due to legal reasons, in this case, we give free shirts to developers.
I had some mails going back and forth with them, as I thanked for the stuff I got and they are lovley.
The quality is good and conformable, the oldest thing I have from them is a Tux-Hoodie I bought two or three years ago. It is still good and has the normal wear signs a hoodie has after a few years of use and washing.