I presume you are referring to the license number for the pre-installed MS Office that came on your PC?
Are you wanting a 1 off or multiple for a business install?
I looked a while back and found Staples was as cheap as anywhere for a copy of the business edition. Our local Asda had the license cards for the home use version as cheap as most on-line outlets (no use for traders though), not sure that they had the business version.
If it is for a small office, then a "proper" open license agreement is probably your best bet