You can buy in bulk 100 ($1.50) or 500 ($9.99) + shipping from a vendor on ebay (uniqupeacking). Here is the link:
http://search.stores.ebay.com/unique...alableQQsofpZ0
These are for size 6 7/16 x 6 1/4 clear envies that are perfect for 6x6 cards...there also have other sizes available. BEST DEAL on the net! If you buy other items from them, they discount the shipping for each by taking $2 off of shipping charge. Fast shipping too!
Hope this helps!