I used this company years ago to cut mounts for an exhibition. http://www.borderframes.co.uk/

I think there was a minimum order of 50, (all the same size) but they did a great job and very well priced. I can't remember the price, but was supplied with 16x20 cut mounts and backing boards of the same archival mount board. I've also used a company in Glasgow called Picturemounts a while back back can't find a reference for them.