I can recommend minifree.org - this is a shop from Leah Rowe, who is the lead developer of the libreboot project. That is a (more secure) bios alternative, related to coreboot.
I bought my Thinkpad T400 from there, some 6 years ago or something. I am still writing on it and i can highly recommend it. However, today i would buy a smaller form factor. so 12,8° instead of 14°.
So it is kind of heavy compared to a macbook air and not the fastest machine, but you can get your stuff done. And it is really really durable, which is the reason i bought one of the older thinkpads.
And with minifree.org you can be sure that the linux/libreboot/coreboot support is really great. Because: since Leah is a developer, she testes everything beforehand and fixes problems when she notices it. So i would recommend to describe what you would want to do. For instance, initially i wanted to use a encrypted harddrive and i had installed the grub variant, but later upgraded to libreoot with seabios. This was much better and fixed the problems with my encrypted harddrive. But i suspect leah would have found out and fixed that already, had i told her that.
Also for instance, seabios has better openBSD support.