Basically the board called a Freebord has 6 wheels. It has 4 outside wheels like a normal board would that act like the edges of your snowboard. It also has 2 castor wheels that are in the middle of each truck. They are slightly higher then the edge wheels giving you the ability to slide the board in any direction just like a snowboard. It's pretty crazy how it works, it takes a little getting used to but if you snowboard you'll learn right away. It's a great way to curb your itch to ride during the off season. Heres a video on how it works for more info.
There are a few stores that sell them but it's not that common to see them in shops. You can get them at Freebord.com or you could get one off of e bay or something. If you have steeps streets where you live I highly recommend it.