If you have an Apple Store near you, buy it from there. They should be happy to help download/install Mountain Lion on it for you. Best Buy or one of the others may even provide this complimentary service.
It's unrealistic to think that all pre-boxed Macs would have it already pre-installed. Now, if you buy a boxed Mac in a month from now, then sure it may well have Mountain Lion pre-installed ... just as my new, boxed MacBook Pro had Lion pre-installed on it upon purchase at a non-Apple Store last August (a month after Lion had been released into the wild!).