You can by "OEM" software by Microsoft from many computer retailers. It is a discounted version of Windows that is sold to be installed on a PC for RESALE. Dell, HP, Acer, etc. are what are called branded OEM, who have special agreements above and beyond the agreement that is included with the "generic" OEM software sold by retailers. This OEM software is NOT for use by the person who purchases it - it must be installed on a PC that is then sold to someone else.
A local system builder would be some person or company in your area that builds PC's, installing OEM software on computers that it sells to its customers.
If you made the mistake of purchasing PC's without an OS, you may be able to "sell" the PC's to a local system builder, who would then install an OEM version of Windows, then sell the PC's back to you. Probably the easier thing - and possibly the cheaper thing - to do would be to return and reorder ... or just purchase Retail licenses.