As Ryan said, I think QEP are the only distributer in the UK and are very reasonably priced. Sets often come up second hand though, so might be worth scouring some of the other Saxo/106 forums before you buy.
Don't forget you'll need management too though - you're probably looking at around £450 for Wayne Schofield to remap your standard ECU, or QEP can also supply you a KMS unit.
For info, if you're going down that route, I might have one for sale imminently