Not many local shops will have a big range of DH bikes as they are quite specialist, not too mention pricey (most shops it seems would rather have a bigger range or low/mid priced all round/general use bikes).
Whether you go for a full-on DH bike with 200mm travel or something a bit less is upto you. Bear in mind a full on DH bike is really only suitable for DH runs and is a bit of a pain for local woods/trails, etc. Hence why many havce more than one bike or go for an All Mountain bike with reasonable suspension 160mm plus that can handle a bit of everything (I changed from a Commencal Supreme DH model to their newer Freeride model which is a bit more of an all rounder >
http://www.chainreactioncycles.com/comm ... -prod83534)
As for cost you are not going to get anything new for less than 2 grand and maybe a lot more, check out YT Industries which make full bikes for silly prices;