Have a fantastic FSA threadless stem .
Have an equally fantastic set of sunlite northroad bars
The SS bike frame and fork (hey..$40 for a factual SS/Fixed frame and fork in cro-mo!)are set up with the 1/18" threadless steerer tube.
The stem is 31.8
The Bars are 25.4
Someone out there make a step down adaptor or a stem for such applications?
Suggestions? (Yes, I am keeping the frame and fork, yes I would very much like to use northroad style bars inverted)

Nitto among others make shims you can place between your bars and stem. I've used them successfully to use 26.0 bars with a 31.8 stem.

You could prolly find some sheet metal and just cut your own for free. My bars are shimmed with some ducting.

Found Em!!!!!!!!!!!
Velo Orange makes a threadless to 25.4 stem (chrome no less!)
Not the cheapest but it's made FOR the application vs cobbling togeather and..it's pretty light in weight as well.
Color me happy D

*cough*

Many, many (many, many) threadless stems have a 25.4 clamp. They also commonly come in 26.0.

All 31.8 stems (probably) are threadless; not all threadless stems are 31.8.

It's likely that your LBS will have a shim (Nitto or not) for three or four dollars.

I have 2 or 3 silver threadless stem that are 25.4 in my parts bin. PM me if you are interested.