The RSI bands indicator as discussed by François Bertrand in his article in his article, "Visual Variation On An Index: RSI Bands," is available for download here.
Once downloaded, from within the NinjaTrader Control Center window, select the menu File > Utilities > Import NinjaScript and select the downloaded file. This indicator is for NinjaTrader Version 6.5 or greater.
You can review the indicator's source code by selecting the menu Tools > Edit NinjaScript > Indicator from within the NinjaTrader Control Center window and selecting RSIBands.