Hi JuneBillie, I got my sleeves at Office Depot. They were quite reasonable for 10 in a package. The are actually photo sleeves and as you can see, Just fit the postcard I made. I will have to be careful with embellishments.
Although I find them reasonably priced, they are more and you get a lot less then the ones online. I will look into those soon. I just needed the envelopes now.