System 76 is a vendor that only sells "laptops, desktops, and servers with Ubuntu pre-installed, and is committed to the ideals of open source software." You can compare the servers they sell. They are also a Ubuntu solution provider so I am sure they can make you a system that fits your needs to a tee.
I used to be a sysadmin at a wholesale telco that ran CentOS exclusively on IBM and Dell equipment. They were just ordered without an OS. Neither company had any problems whatsoever with honouring service contracts (and we used to call them fairly frequently for warranty part replacements and so forth). Keep in mind though, that we didn't want software support.
So long as you're ordering them without an OS, and you're ordering actual servers (so you don't need to ring the desktop support line) you shouldn't have a problem with any of the big vendors.
I know this is an old question, but for the benefit of people who find it years later like I did - ZaReason is a small company that sells Linux desktops/laptops/servers, and I've had good experiences with them (I got a desktop to be used as a small data analysis server at work, and a high-end laptop to work on). They offer a choice of many different Linux distributions. You can get an extended warranty and support, too.