They don't have them available at the post office. You have to order them online and they get delivered at no charge. I don't recall ever seeing them at any post office anywhere.
There presence is totally dependent on the postmaster in charge. A lot of them are lazy and don't keep all the items stocked that they could. I have a post office in a very small town about 5 miles from me and the lady that runs that post office has every box, bag, and letter that USPS offers in her post office. Just go online and order whatever you want for free with free delivery.
