Last night I was able to get one, but I had called several places around the country in a search. This is what I was told - If you had spoken to the store on Monday and they were holding one for you, then you were fine yesterday. However, the order came down from the Mothership to return all EOL Mac Pros, and the stores were complying with the order - boxing them up yesterday for shipping back today. Unless you could physically get into the stores to purchase them before they closed yesterday, you were out of luck. There were several stores that said they had at least one in stock and would have been more than happy to sell it to me, but only if I could physically get into the store since they can't technically do a purchase over the phone.
I'm sure some stores will be different than others, but most of what I've found in the West has been pretty similar in situation and explanation.