Bloomberg is a good place to start when looking for data for fixed income securities. Use either the CORP or GOVT shortcut key for corporate or government securities research. Bond Market News (TOP BOND) or World Bond Monitor (WB) provides comprehensive news and data for the global bond market.
Mergent's BondViewer provides information on issuer and bond level terms and condition data, US taxable bonds, municipal bonds and retail notes.
Wharton Research Data Services (WRDS) provides access to CRSP (Center for Research in Security Prices) data. Part of the CRSP package is the U.S. Government Bond File, which contains data for approximately 3,100 marketable U.S. Government Securities.
Eikon provides time series data for a wide range of global fixed income securities.