You've got a bit of a challenge on your hands. Everyone wants a Purdey or a H&H Royal, so finding a decent one at a fair price is not an easy thing to do.
It can be done, though.
First, though, a few questions:
-Do you know your measurements? If you don't, I would head over to G&H for a fitting. Once you have your measurements you'll have a much better idea what kind of gun is right for you. Restocking a Purdey or H&H Royal is expensive ($5,000+), so you want to buy a gun that measures up to your dimensions.
-What length bbls do you want? Some guys want 30", some are OK with 28" - 30".
-What kind of shooting are you going to do the most - upland game, sporting clays, waterfowl, pigeons? If you're going to shoot a lot of sporting clays, I would want a heavy proof gun in the 7.5 lbs range. If you're going to hunt grouse and pheasants, a standard game gun will be just fine.
As for places to find Purdeys & H&Hs, I would concentrate on the dealers listed at doublegun.com and here at doublegunshop.com.
Most of the dealers in the US are on one of these two sites.
Other guys to check out are:
http://jaquas.com/http://www.finefirearms.com/I've dealt with Jaquas and they seem to be OK. I've never dealt with the guys at Fine Firearms, but he has some nice stuff.
I would also look for guns at gunsamerica.com. There are a few Purdeys there now that may work for you.
I would also watch the US auction houses, especially these guys:
http://www.juliaauctions.com/This Purdey was in there last auctions:
http://www.jamesdjulia.net/firearm/oct06/catalog_detail.asp?Details=60727It did not sell. It may be back this spring with a lower reserve. I checked it out and it looked like a nice gun.
The biggest think you need to do is enlist the assistance of a gunsmith who is an expert in English firearms.
Before you buy any gun, you have to have this 'smith review and inspect it. This is extremely important - I cannot stress it enough. Do not buy a gun without having a 'smith look it over. Do NOT trust a dealer's or auction-house's evaluation of a gun. I don't care who it is - don't do it.
If you want some more info, drop me a PM and I would be happy to help you out.
Good luck.
OWD