Oh, they are sold as 'Sets'. That was unexpected. I ordered two long ones, the short ones are indeed sold-out. Luckily I have a pending order and could add them 8to avoid the shipping costs), but that also means I have to wait a month before I can finally test the sbrick.
The short ones are sold out throughout European shops, I guess US has its own warehouse. I had few pieces of the short variety in basket since December, waiting for the January items to appear, and around 2nd January they sold out. Who knows if it's just coincidence or SBrick owners finding out they need those :D
I don't know where you live but http://shop.lego.com/en-US/LEGO-Power-Functions-Extension-Wire-20-8871 is the long version and http://shop.lego.com/en-US/LEGO-Power-Functions-Extension-Wire-8886 the short one. You can find it at "Categories" -> "Power Functions".
Now that you have a desgn for the PF connector already, it would be awesome if you could offer custom PF cables. The original lego ones are not easy to get at bricklink right now and are quite expensive (8€ for a short one). I won't even need the old 9V adapter feature.
Right now I've tested my SBricks with a custom wire, but I can't put it into any of my models as I miss the cable :/