The QMs dept in Blandford might be a good place to start. Alot of the guys leaving the corps apparently hand in their old dress to sell on for them. Im sorry but im not sure exactly who it is running with it but the clothing store might be able to help.
The only reason I know about it, is when I was buying mine a few years ago it was suggested to me but I went for brand new in the end.
This is where I got mine from and they do a good deal. He will come to you and measure you up, go away and make it to measure then deliver it personally with any "ancils" you need, they will also do all your medals, court mount and all. Pretty much a one stop shop. (better get some commision out of this!!)
If your lucky enough to be substansive after 31st March of this year you are also entitled to the 450 ish quid off the army to pay for it. Cash in hand as well if you are lucky to have an imprest clerk on your camp.