personally I go for pre cut/mitred mouldings from either from Arqadia or Lion and buy glass the size I want. May work out a little more expensive but you are not limited to ready made sizes and you'll get better service than dealing with neilsen direct who, the last time I tried, had admin staff who were completely incompetant. I think it took 6 calls re-explaining the same thing over and over before they finally understood the mistake which was theirs in the first place. I will never deal with them again.
Look at http://www.arqadia.co.uk or http://www.lionpic.co.uk/
both the above deal only with trade but you are a pro photographer so that counts OK. Request a catalogue from both of them. Arqadia`s aluminium mouldings are excellent. Lion supply absolutely anything you want for framing.