Landsat data does not come with RPCs nor is it an appropriate sensor to use with the Build RPC tool in ENVI since it is not a pushbroom sensor.  However, most of the Landsat data you can download from the USGS is terrain corrected (orthorectified) and is of very good quality.  I would suggest you look for data that is terrain corrected from the USGS.  The entire archive is free now. http://landsat.usgs.gov/