Hi Dai, welcome to the forums!
All of Underworld's albums are available on iTunes:
http://itunes.apple.com/us/artist/underworld/id996876
Otherwise, most of the rest of their material is currently out of print, so you would have to buy it on the secondary market.
As for the "Ooooh Ron , Oooh Ro-on" song, my best guess for that is "Spoonman":
If that's the one, then it looks like Dubnobasswithmyheadman is the album for you to buy.