I think Bob Roberts USED to carry them, although I believe I bought his last set a while back.
Ebay is your best bet. They are ALWAYS on ebay. Both used, NOS, repro ones (which are not quite as good), and can often be found in complete control panels, which is often the best way to pick them up (just look for a butt ugly control panel with a pair of red balltops on them, 9 times out of 10 they will be Wicos.
They were also available in other colors, but all the other colors are super rare.