Mike, let's start from the begining; what router do you need a collet and collet nut for?
That's the problem. I'll tell you, but I've seen this slammed on this forum before.
I have several 1/2 inch routers, several 1/4 medium duty routers, 2 trim routers, 2 separate router tables... And this is not a router.
It is a Montgomery Wards Power-Kraft Radial Arm Saw with 2 auxiliary shafts. One shaft turns at 3240 for sanding, buffing and grinding. The other turns at 20,000 for routing and shaping. The thread on that shaft is 15/32 20 tpi.
There was a router adapter that used to sell for it (1/4/ shaft bits).
I used to use it as a RAS, drum and disk sander, buffer... I was given this as it didn't work. Rebuild it (brushes and bearings). I am now changing bearings again. So yes, it's had a whole lot of use. I have it tuned, as with the other tools I use.
I sue my router tables allot. I used my free-hand routers with lots of jigs I've built. Instead of building a tilt router lift for one of my tables, I am toying with more of a pin router / non-computerized CNC type of capabilties. I looked into the Maxis tooling system... Around $10,00 plus. (Ouch) As with all my jigs, I make do with what I can afford, giving me the ability to what I need and what to do. Retrofiring my RAS seems to be an answer.
Montgomery Wards has been out of business for 10+ years(?). I hear that Tool-Kraft had been selling some repair parts... but don't know about "options."
Still, that would limit me to 1/4" bits. Finding another chuck would give me the ability to use both my 1/4" and 1/2" bits.