There is usually a couple around but not much choice in the UK. There are a couple of RT's on the UK club site
http://www.ukviperclub.org/ and a few GTS's on autotrader, don't forget the Euro models are badged Chrysler so search for that also.
You may need to import if you have any specific requirements re colour/year. Head over to the UK forum
http://www.pistonheads.com/gassing/f...p?f=76&h=0 as there have been a few new imports lately so plenty of experience to advise you.
Yes they are pretty rare on UK roads, Dodge are only officially importing half the number(15) of SRT10 than Ford are GT's. Quite a few more GTS's and RT10's around but even so its not often you bump into one, even around here and there are a few known daily viper drivers near me.