Section 508 certification doesn't exist. Voluntary Product Accessibility Templates (VPAT) are the best way for vendors to provide accessibility information to the government.
Some people are claiming that they can provide vendors with a third party certification that their products are Section 508 compliant. Well that simply isn't possible! No one, including the agency buying the product or service, can certify compliance or conformance, because there are no accepted tests. In the end the federal agency purchasing a product or service is responsible for determining if the deliverables meet their requirements.
Vendors can give the government information about the accessibility features of their products and services in the form of a VPAT. The government looks at VPATs during their market research, and may even require vendors to provide VPATs or a Government Product/Service Accessibility Template (GPAT) for a proposal or with deliverables.
That being said, there are a lot of experienced companies who can help vendors prepare VPATs or GPATs. Many companies also offer expert services to federal agencies to assist in determining how well products and/or services conform to the Standard. You can find some of these companies through Vendor Accessibility Resource Center.