the best place to see where has what available is to watch the "octopus availability" thread.
As Simple mentioned, though, we've often found that people want dwarf octos because the smaller tank needs seem convenient, but because they tend to be nocturnal, shy, and short-lived, people are sometimes disappointed with them compared to larger octos like bimacs and briareus... just food for thought.
For more information, please read dwhatley's thread called Trapper's Babies - this is one of the accounts of keeping O. mercatoris, the most commonly available dwarf.
A number of people here have kept this species, but Monty and simple are right, these little octopuses are reclusive and nocturnal and many people prefer to buy larger tanks and have more outgoing octopuses.
I just want to emphasize the comments about the dwarfs not being for everyone. SOME of mine interact and one is a joy BUT I am a late night person and have spent countless hours working with my little fellas and they only started to interact at around 5 months old. I want to emphasize that just to have some interaction, I have worked with these shy, night time creatures NIGHTLY for the last nine months. With their WC mother, I would fall asleep in front of the tank, sometimes without even seeing her. Many people have reported only being able to catch glimpses of their dwarfs using a red flashlight, late at night. I have one that only recently started coming to the front of the tank at feeding time. Neal laughs at me when I "crow" about my success with my crew. Rolling his eyes, he reminds me that not everyone stays up until 3:00 AM.