They are available for purchase now. I have one, I bought it a few weeks ago. I have seen them instore at Gamestop (2 different ones), Best Buy, and Wal-Mart. They are also available online some places too, like Activision.com's store.
In the store, be discerning, the box looks pretty similar to the full game/guitar package and can easily be overlooked.
The retail on the official guitar is $69.99. That's what you'll pay at all retail outlets. If you can find a cheaper one, more power to you. And yes, $69.99 is a rip-off if you ask me.
If you have patience, Nyko is releasing a guitar too that they claim is due out this month. I haven't seen it anywhere for sale yet, but check www.nyko.com for details on it.