I've always found that the plastic pocketed QSL displays work well and will hold about any sized QSL card, from anywhere. They hold 40 cards per display, and at one time I had 50 of these displays making wallpaper all the way around my ham shack (all four walls), to display 2000 cards. It's easy to change the cards displayed without taking these things down, if you're tall enough to reach the upper pockets!
Available from:
http://www.artscipub.com/73 de Steve, WB2WIK/6