PreciseBits is supposed to be coming out with some 3/16" collets.
But generally, for most routers you'll only be able to get 1/4" and 1/2" collets, and 1/8" if you go with PreciseBits.
One reason that a lot of those Onsrud bits are so cheap is because you can't use them in a router. Buyer beware. :-)