The knobs are manufactured by Mentor, part nr. 505.6131 for the 12mm round knobs, part nr. 506.6131 for the 17mm round knobs and part nr. 5581.4631 for the 17,4mm wing knob.
available at several distributions such as Farnell, Distrilec, Conrad...
not cheap, but nice looking

maybe you'll need to trim the shaft lenght of your pots for the 12mm knobs, because they're just 12mm long.
in the germany area you'll get the best price at Buerklin.de
cheers,
Sub
ps. the wing knob must be drilled to 6.35mm to fit on the grayhill encoder. (I have used a 6.4mm drill)