Is there any particular reason mobo standoffs are made of brass?, if not can anyone tell me where to bet stainless steel ones.

For their tensile strength with small diameters?

No idea, cheaper perhaps. Brass conducts, which is the only thing that standoffs need to do I believe. It doesn't have any major differing properties than other metals at first glance.

My NZXT Zero came with some sort of steel standoffs, but they were proprietary - stubbier and larger diameter.

machining properties and cost.

I work for a company that manufactures fasteners for all types of computer componnents all types of materials are used from plastic, steel, stainless, brass, aluminum, and high carbon steel which is heat-treated to hardend and or [plate]. you could use anything.

so does anyone know where get stainless steel ones.

One of the reasons may be that there is apparently only one major supplier of standoffs - StarTech.com. Virtually every reference I found on the web referenced them. However, my persistence paid off for you. Keystone Electronics makes them in stainless steel. Unfortunately, they are a wholesale manufacturer. Allied Electronics does carry them for $1.57 each. WARNING: the link I posted is for the .375" height standoff - I haven't measured a standoff lately so make sure to order the right height part.

EDIT: BTW what you are looking for is referred to as a jackscrew standoff #6-32 to M3. #6-32 referrs to the male thread that inserts into the chassis and M3 is the interior thread size of the female fitting. All of this information was obtained using Google.