US government contracts

http://www.fbo.gov/ has over 12,000 US government contracts in excess of $25,000.
You have to be registered with central contract registration to win these contracts.

http://www.gsa.gov/ US General Services Administration gives details of subcontracting for government contract winners.

State government tenders

http://www.findrfp.com/

http://www.bidcontract.com/ gives details of federal contracts as well as town and state contracts.

http://web.sba.gov/subnet gives details of us subcontracts