Where can I buy this module?

I would like to consider developing with this MC does it exist for sale yet or is there a release date?

You can see a list of currently supported boards here : https://www.mbed.com/en/development/hardware/boards/

While mbed does not sell boards you can buy them from the hardware vendor who makes them. So for example the frdm-k64f board would be bought from NXP.