Recently I shared how I made arm warmers for my kiddos. Of course, I used tights for ours but knee socks work also.
Here are some places that I found while browsing that carry cute knee socks...
Sock it to Me
Little MissMatched
(aren't those cool and FuNkY?)
Gymboree has some pretty good sales going on right now. You can get some solid colored knee socks for just $1.99. Stock Up! You can always embellish them with buttons and patches.