Ok Zman, took a while to pull the collet of my RE-600 (had to first remember where I stashed it during workshop re-organisation).
With reference to the attached photo, are you looking for (1) or (2)?
If (1) you need to get the nut and collet - the collet snaps into the nut, but being relatively delicate, I would not mess with it. It looks identical to those on my Makitas, and I imagine they are interchangeable. The RE-600 only came with the 1/2" collet - for other shaft sizes, you need an reducer sleeve like (2). These are generic, and in fact have to be: the outer diameter is 1/2", the inner diameter is what you require: 1/4", possibly 3/8", 6mm, 8 mm. Some routers for the EU market now come with a 12 mm collet, and reducers for the metric sizes. The 12 mm and the 1/2" collet-and-nut combinations have been interchangeable in my experience so far, but obviously you cannot use an Imperial reducer in a metric collet, or vice versa. I have a 40-year old Hitachi, which can take a 1/4" or a 6 mm reducer in its 8 mm collet, but I think that versatility has gone out of fashion, probably with good reason.
I don't remember the names, but in the USA you have a number of specialist stores that stock such spares. Perhaps some of the other members can do the aide-memoire shtick?