Here are some good sources to check:
Fidelity for looking up stocks, mutual funds, indexes and annuities.
QuantumOnline for running a quick search using ticker symbol, cusip number or name. It provides symbols and cusip numbers for stocks and mutual funds.
Atlantic Financial for looking up cusip numbers of some mutual funds.
MSRB for looking up cusip numbers for municipal bonds using their Muni Search.
CUSIP Global Services for all types of securities. For a fee, you can access the entire universe of CUSIP numbers (Stocks, Mutual Funds, Corporate, Municipal, Government, Mortgage-Backed and Private Placement) and ISIDPlus - the global cross-reference database.