The importer number (requested on Customs and Border Protection (CBP) entry paperwork) is usually the IRS business tax number assigned to businesses.
If you do not have an IRS business tax number, you may use your social security number in any paperwork requiring an importer number.
If you do not have either an IRS business tax number or a social security number, you may request that an importer number be assigned to you by filling out a CBP Form 5106 and presenting it to the Port of Entry in your location where you will be filing your customs entries.
Please press here for additional information from CBP's website.
An explanation of the regulations governing the issuance of an importer number can be found here.