I, too, have used redimat with great results. They stock a very inexpensive mounting/backing board that, though it has a grey core, is acid free and worthy of not overly expensive prints. They also have a complete array of the best rag mat, and crescent select board at excellent prices. Since I cut my own windows, it's great to have good prices on blanks (which I detest cutting, 'cause I always mess up the full sheets into 16x20s and such).
For the selection and pricing they come highly recommended. I prefer to not cut so just get pre-cut sized. Usually 18x24 for prints on 13x19 paper but for those that do cut their own the blanks of all sises are available.