you have a few options when it comes to using circular needles for a sock. There's the Magic Loop technique, as well as 2 circ technique, but there are 3 more options that I know of. Clover makes a 8" circ, but they're becomming more and mroe difficult to find. From what I've read, they're just too akward because of how short they are. There are also 11" stretchy circulars for sock knitting. http://www.stretchycirculars.com/index.html
, and you can also use a 12" circ. the 12" circ can be difficult to get your stitches around for the first few rows, but it works fine for the rest of your sock. using any of these last 3, you will either need another circ or dpn's for closing the toe.