You could always just buy plain flat cards and fold them yourself. If you ask a diemaker who makes cutting dies for the packaging industry they can make you a board with a creasing rule set vertically in it. You then set the card on it and run a creasing tool over it. A creasing tool is just a small piece of wood with a notch in it. I've no idea how much they'd charge you for it (probably not much and they might even make it for free) but it might be cheaper in the long run than buying pre-folded cards and besides, flat cards would be easier to print on. I don't know your location but you could try Laserform at
http://www.lasercomb-dies.co.uk/ as they have offices in Redditch and Leeds.