You did not specify if you need the index or the constituents. I just purchased the individual symbols of the S&P 500 in a minute frequency last month from Pi Trading.
http://pitrading.com/historical-stock-data.html
If you need the actual index...
I will never understand why DTN is under-rated compared with eSignal. I find their data feed to be superior in every way. Only negative is you must supply your own charting software to take full advantage.