I have three Stanley Routers, but none of them have an ˝" collet though. I purchased spare collets 1/4" and 3/8" on American ebay.
Bosch Power Tools bought Stanley Power Tools sometime during the early 80's, and I think there are even som models that are branded Stanley-Bosch or vice versa, just like the Stanleys once were called Stanley Carter when Stanley had bought Carter. There are rumours that some parts from Bosch routers from that time are interchangable with Stanley parts, but I have found no one who knows anything about it at Bosch here in Europe though. Perhaps Bosch America knows something.
The last resort, if you, like me are fond of your router and really want to use it, is to get in touch with someone having a machine shop who either can make collets and nuts for you, or convert from a suitable other brand.
I like to use my Stanley routers, but they are not options if I would have just one router.