I don't know of any specific sites for searching for a 999 - I won't recommend Ebay
Maybe the good folks of DSC will keep an eye and an ear out for you.
The 999 new in the UK is about £11250, the 999S is about £13950 (pounds sterling, not sure about euros, etc). Although the S model costs about £2700 more than the basic, you get a load of extra goodies for the money. Bike Magazine did a costing for converting a basic model to an S. If I recall it right, it was over £3500 for just the cycle/suspension parts - without even the cost of modifying the engine.
Good luck in your search.