Khuxford - my PO does not keep them in house either. I order them (for free!) online and they come in packs of about 10.
I print my sending label from my account on the USPS site (cost is only $5.70) but it has to ship within 24 hours. (postman will pick up with the regular mail)
You have to get the return stamp at the PO for $5.95 - it has no expiration date.
All this saves the hostess from having to spend hours at the PO to ship everything back to everyone who participated.