I get my gold and silver mirror card from papercard.co.uk. (sorry I don't know how to do a link)

. If you use loads their price works out really cheap, e.g. approx 250 A4 sheets of Silver at £30 (inc. vat.) and approx 300 A4 Gold at £36 (inc. vat). They call it a 2 ream box, the Silver is thicker card than the Gold, which is why you get more sheets. So if you use as much as I do (250 sheets since Dec.) they might be worth a gander.
Karen x x
p.s. there should be a www. before that site name, but every time I try to type it in and preview the post a whole load of gobbly-gook comes up. Needless to say I'm pants at this PC stuff