In the UK try Slater's Menswear. I am sure they do dinner suits from about £100 and decent ones for about £200. Also a good range of evening shirts at particularaly good prices.
They don't have a website but here is a review link for them :
I know they have shops in Glasgow, Edinburgh, Manchester and some other main cities. If you get the number for one shop, just phone up and find out if there is one near you.