Buying Individual Stocks
You can buy individual stocks listed in the S&P 500 from any major online or full-service brokerage. Lists are widely available online and from Standard and Poor's. Many of them include ratings for each stock listed.
S&P Index Funds
You can also buy an S&P 500 index fund, a pool of all the stocks in the index. The managers of index funds don't actively pick stocks but simply attempt to match the contents of the S&P 500 list. Many fund management companies, including Vanguard and Fidelity, list available S&P 500 index funds. You can buy these through a broker, just as you would any mutual fund.
S&P Exchange Traded Funds
Many management companies sell S&P 500 exchange traded funds, pools of S&P 500 companies that trade like individual stocks. Some of these ETFs are true S&P 500 index funds, while others represent some market segment, such as all value funds or all growth funds in the S&P 500.