You may have come across this already – and if you have, you are on the right path. Here is a solid list from the coreboot docs on mainboards they support.
You should be able to derive a few vendors from this list, as well as compare any possible candidates you have found to it. Some vendors on this list produce hardware that is Linux-forward, such as System76 and Purism. Linux-forwad vendors are good place to start.
System76 has laptops and small profile desktops, servers, etc. Purism has most of that, as well. Both are gonna be a little pricier than a typical vendor, but the hardware – and their respective missions – are worth it if you have the means. I have owned and / or used PCs from both these vendors, and they have been quality.
Otherwise, that linked list above will be helpful to determine if what you are researching has a mainboard that works. Best of luck!
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