-- Last Updated: Dec-13-12 12:52 PM EST --
Trout is probably right that wing nuts and lock nuts are mutually exclusive categories. But you could give McMaster-Carr a try, they carry a lot of hardware.
www.mcmaster.com
PS indeed they have them: "Nylon-Insert Locking Wing Nuts", on page 3208. However, the smallest size is 10-24 or 10-32.