Socks are an item available to buy at Kicks, a shoe store that is unlockable to be to the right of the Able Sisters store. There will be two sock type items available in his store each day once it is built. You are not allowed to try socks on, but you can get a basic idea of the look you will get from how they are presented.
If you are looking for a specific style, press ctrl-F on Windows and ⌘-F on Mac. From there, you can type in the style you're looking for, and be able to easily use the forwards and backwards buttons to go through your options!