Depending how big a purchase "in bulk" implies, you might also be interested in http://www.cards4magic.co.uk/acatalog/C ... _Etc_.html
Total Merchandise charge 32p each when buying 500, or 26p each when buying 1000. Those prices exclude VAT, so they're actually 37.6p and 30.55p.
Card4Magic charge 33.3p each when buying in multiples of 6. And you get the option of having one of the pockets opaque, if you so wish.
On the other hand, Total Merchandise include personalisation in the price.