I'm a HUGE craft addict and get a variety of different sized card blanks and envelopes from Craft Creations ...
http://www.craftcreations.co.uk/page...l?category=185
They have a white envelope which is 64mm x 99mm to fit a small gift tag sized card in two qualities ... standard and best.
You get quantity discounts but looking at it they're going to work out more expensive than the place Fee suggested.
Best of luck