Jim's name comes up usually within the first few posts here on this subject. I've never used him but many have happily had him build their computers. He offers a pretty good deal.
I would like to see some competition though. Either that or make him the officially recognized Cakewalk computer builder.
I know there are others who do it as well. We just don't hear about them as much.
I think most of the big name daw builders are over priced for the tech they offer. JMO YMMV. They usually drop back a generation or two to assure there are no issues or offer a minimum build for your needs at a higher price. Chips and builds having been on the market awhile are proven and less of a liability for them.
At one time there were more fixes that could be done in software that would streamline a machine for daw work. With more recent OS and better hardware performance many of these basic issues are now moot. At one time I think you needed to lean more on a pro computer builder who knew what needed to be done. Much of this was more software related than hardware related. I don't believe we need that as much anymore.
Many report great results with high spec'd mass factory built computers. Even the basic ones can record smaller projects with no problems. The hardware in mass made machines is reported to be of less quality in many cases. How this actually plays out in terms of years of reliable use is anyone's guess. Especially if you have a good replacement repair warranty in place.
While I would recommend you either have a better machine built or build it yourself, this isn't always mandatory all depending on your needs. Of course buying the bottom tier machines for audio work is always asking for trouble.
I built my own after some brief research and I saved a lot of money in doing it. For me it's just assembling parts and loading the OS. It can potentially get complicated, but much less so if you use good hardware.
I don't want to be the one to tell you to build your own, because all of those who bought from Jim or another similar will say, " I told you so" if something doesn't go right. I was willing to take that chance and it payed off for me. For those not technically or mechanically mediocre a builder is probably best with a higher spec'd factory computer being a second option.