Several ways to get a PC without an OS on it really. Build yourself
from scratch / bare-bones systems which have most parts in place, just
need to add in a HD / some hardware vendors supply ready built full
systems without OS's too.

Just as a FYI - most vendors don't pay anything like the price we
would have to for Windows. They use volume licencing deals which cut
the price of Windows to almost nominal compared to the rest of the
parts and cost of assembly / testing / shipping. So really you aren't
paying that much of a premium for Windows and erasing it isn't a loss
of a great deal of money - just a case of whether even that small
amount going to MicroSoft's coffers rankles or not