I found these at the Vintage Paper Fair for only $1 each.
Wednesday, May 29, 2013
Have you ever heard of these first day of issue envelopes? They were used to commemorate the first time a certain postage stamp was available. The cancellation is from the city where it was first issued. Since these have been canceled, you'd have to put proper postage on to mail them. I might just use them as an inner envelope so they don't get mussed. I found these to be particularly great: