Yahoo Finance, any brokerage account charting tool

RSI is just an indicator. Any common charting tool can show the RSI of any stock/ETF/MF. 

Typically, when RSI approaches 30, SPY is at a low point, but it is definitely not the lowest point.  A lower low will occur within the following few days/weeks.  Starting DCA-in process at this point will often incur paper loss in the first few days. Weighted DCA-in can ensure that more money is put in at the lower point and thus ensures better gain when market eventually turns around.