Well, welcome to the forums then. Between 2004-2008, Underworld did a number of live radio show broadcasts, where they would often play unreleased tracks and live renditions and experiments of already-released tracks. None of these have really been made available for purchase anywhere, but fortunately, we have recordings of the shows. Most of them have been captured in their original broadcast format, usually low bitrate MP3 or AAC.
These can all be found to download via BitTorrent on the
RTSR Tracker. Just register and your account should be approved within a couple days. Check out the "Radio Shows" category.
As for why tracks from the RiverRun era are not currently available to download? Well, the band are aware, but, for whatever reason, have chosen not to make them available. It's possible that they're all planned to come around again with a RiverRun super deluxe box set or maybe on an Oblivion with Bells box set. We're not sure yet though if that box set series is going to continue though.