https://www.google.com/webhp?ie=UTF-8&rct=j#q=spx ya can’t buy this index, however, you can buy following ETF at the correct signal.
http://stockcharts.com/h-sc/ui?s=SPY&p=D&yr=0&mn…
http://stockcharts.com/h-sc/ui?s=SH&p=D&yr=0&mn=…
http://stockcharts.com/h-sc/ui?s=SSO&p=D&yr=0&mn…