I was recently en route to the airport when I stopped at a local post office to obtain postcard stamps. The lady was able to find only three of them. I said, "The Postmaster General is going to hear about this." and I'll bet she thought I was kidding. I was flying to DCA and while in Washington, I did stop at 475 L'Enfant Plaza and went into the joint to make a complaint. Some bureaucrats came downstairs to write up my complaint. Earlier that day, a post office in Alexandria, Virginia had only coils of 100 of postcard stamps. If a tourist wants to send 9 postcards, the tourist should be able to buy nine postcards stamps.
It's not the fault of the individual post offices. They are only allowed to have limited amounts of stamps on hand. Shortages are also very common. The post offices have no problem of placing orders for new stock, but the distribution system is one big snafu due to changes in the last few years. There use to be many local distribution offices for stamps for each post office, but since it's been centralized it's slow.