I called Galaxy Products that Paul linked to, and I was told they don't carry that particular collet anymore, and don't make them anymore either due to lack of demand. Elaire doesn't carry it either. Neither does ereplacement. The Bosch 1617EVS collets won't fit the 1615EVS - I have several 1617EVS, and I've already checked.
I found this website, Radwell International, Inc. that sells it (I can't post the URL). I didn't go thru the complete checkout process, so don't know for certain if they actually have them in stock.
It's $42.60 plus another $12.42 for shipping, so $55.02 total for 1/2" collet. Right now I'm not sure if it's worth it to spend so much to be able to use this router. The router looks like it's been heavily used, and some important parts are missing (speed control dial, locking lever to lock the trigger, and depth gauge), though the router still works. It's also louder and sounds much more powerful than any of my 1617EVS routers (3.25HP vs 2.25HP). I was hoping to use this in a router table for raised panels, but I'm not sure how easy it would be to adjust the bit height since it's a plunge router. Does anyone have experience with the 1615EVS in a router table?