I found this really good supplier of Daler Rowney mount board and Meg and I used them for the last comps. 10 x A1 sheets (which mount 2 prints each sheet) are £24.95 (rrp £49.50) plus delivery from £1.95 (or £3.95 tracked). We have been using Snow White Textured. Free delivery over £30.00. You have to buy 10 sheets of any single colour.
These are normally £4.75 each at hobbycraft ie £47.50 for 10.