Many federal agencies and large primes contractor Small Business Offices maintain their own In-house Vendor Databases. Here are several to discover.
Federal Agency Directories
EPA | Small Business Vendor Profile System (SBVPS)
NASA | Vendor Database (NVDB)
Federal Reserve System Supplier Diversity Program
SEC | Diverse Vendor Database
NOAA | Small Business Profile Registration System
Large Prime Contractors
Small Business Liaison Offices are the primary entry points for small businesses wanting to subcontract with large primes. We recommend that small businesses develop experience as a subcontractors to other smalls and win a prime contracts within the federal government before trying to get on the team of a large prime.