Today, being April 7, 2011, I wanted to get the 29-cent Herb stamps featured in this thread. I finally found them (half sheet scanned below):

However, it certainly wasn't easy. I went to FOUR post offices before finally getting a couple of sheets (and that was just by chance):
Post Office #1 - Didn't have any. "Maybe" they will be in their next order that comes in Monday. (Still using 28-cent and 1-cent stamps to make up the post card rate at this point.)
Post Office #2 - Never HEARD of it. Wrote it down to check on, as they never even heard of the stamp, never mind having any in stock!
Post Office #3 - Didn't have any. Suggested that I was mistaken and they maybe they are being issued on April 17th. I told her to check her "Postal Bulletin"...she didn't have it.
Post Office #4 - Clerk told me he never heard of it and therefore didn't have any. The clerk behind the sales floor heard my request and said he had some and gave them to the window clerk to sell to me. He said no one has come in asking for it; so they didn't open their supply stock of them until I made the request.
Apparently the commemorative stamps are more likely to be found on the "first day of release"; the definitives are a bit more elusive to find at least on the first day of issue.
Anyone else have a similar post office experience?