The OwB singles appear to be out of print. It was on some of the digital single versions, but they are no longer available.
Your best bet is probably to find one of these promos, but I wasn't able to find any for sale off-hand:
http://www.discogs.com/Underworld-Be...elease/1463793
http://www.discogs.com/Underworld-Be...elease/1464598
http://www.discogs.com/Underworld-Be...elease/1162087