Knob for the Gil Hebard spotting scope mount?

I lost mine and trying to figure out where can I get a replacement. Does anyone know a good place to shop for it? And what is the thread size?

http://www.larrysguns.com/Products/Knob-and-bolt-for-Gil-Hebard-scope-holder__HSH-RK2.aspx

Perfect, thank you so much, Chris, this is exactly what I needed!

If you have a good local hardware store, They will have a selection of black plastic knobs in a variety of sizes.  Take your spotting scope with you to match the threads.  It's a common thread, don't know it off hand.  Probably a couple of bucks.  I use them for a variety of projects.