The U.S. government's central source for lobbyist disclosure information is Ethics.Data.gov, which provides access to White House Visitor Records, Office of Government Ethics Travel Reports, Lobbying Disclosure Act Data, Department of Justice Foreign Agents Registration Act Data, Federal Election Commission Individual Contribution Reports, Federal Election Commission Candidate Reports; and Federal Election Commission Committee Reports.
The Center for Responsive Politics posts a database of lobbyists on OpenSecrets.org. More information is available through CQ MoneyLine. If you have access, even more detailed information is available using CQ's subscription-based First Street.
The U.S. Senate Office of Public Records posts Federal Lobbying Disclosure Act Reports.
Rankings of the largest lobbyist firms are published in Influence magazine and the National Journal.
Columbia Books produces lobbying-related databases, mailing lists, conferences and publications (see Lobbyists.info). Their Washington Representatives Directory is a hefty 1-volume print compilation of lobbyists, lobbying firms and clients. The book provides contact information, brief bios and several indexes (including firms and clients by legislative area, clients by subject/industry, clients by country).