Libre Computer makes a range of SBCs and supports mainline Linux. I use their products, and I'm very happy with them.
"Most ARM single board computers depend on proprietary binary blobs to boot or ship with outdated vendor kernels that are never upstreamed and quickly abandoned. Libre Computer takes a different approach: we fund and contribute to the mainline Linux kernel and U-Boot directly, ensuring our platforms run on upstream open-source software with minimal proprietary firmware."
Yup, seconded! Libre Computer do a great job on this front