Is that a Garelick seat?  You could try contacting them to see if they sell that separately.  But, really, you don't need anything special.  Any old knob will work, right?  Try a local hardware store - a mom-n-pop type place - my local one must have 15 different styles of knobs... all about $5 each.  Heck, I even once made one out of a piece of a starboard - cut it into a circular shape and knurled the edge for a little grip.  That was a freebie as I had the starboard laying around and needed a small knob for a confined area.