they are available, but pretty much only on eBay.
moving from threaded to threadless as the "standard" happened at almost the same time as the transition from many bikes being 1" to being 1 1/8" (before anyone *****es i'm talking about the majority of bikes, not EVERY bike). this means that there was only a small window (in the history of bikes) that 1" threadless was made. just as rare is 1 1/8" threaded.
as tejano said 1" stems that fit 25.4 and 26.0 clamps, but you won't find 31.8. by the time 31.8 bars came along, 1" threadless stems were practically obsolete. if you want to go with a 1" you will have to take what you can get, if you get a 1 1/8" with a shim, the world is your oyster.
and if it helps, it's basically impossible to tell and it doesn't weaken your system any (provided you are using the correct shim)