From Elaire Corporation.
http://www.elairecorp.com/makitaroutercollets.html
(supported models include: RT700C, RT0700CX3, RP0900K, 3621, 3621A, sizes are 1/8", 3/16", 1/4" and 4, 6 and 8mm --- sets are available)
These became available 'cause a fellow Shapeoko user put together an initial order which got the ball rolling (and a custom die made?)
I bought into the initial order to support it, though I've been using a Harbor Freight 1/4" Trim Router as a spindle on my Shapeoko --- but will probably go ahead and buy myself the Makita this weekend.