you can get it in halfords but not sure you would want generic brand in the viper, they may do a branded one also though. Or you could take out a mortgage and put in the castrol syntorque which you can get from Chrysler's, see the wiki at the top of the thread for part numbers.
I've been using Red Line fully synth ATF which is equivalent to Dexron IV. I use Opie Oils who do club membership discount for the Corvette Club, I'm sure they'd do the same for us.
Thanks for the answers. In the end I've put in Mobil1 Dexron III. I guess I was really asking if anyone has tried a higher Dexron rating. I won't be able to feel/test my gearbox after oil change until I can get it back on the road, which will be probably some time in April.