They aren't rare. Any bike shop should have them. You'd be better off getting it from them anyway since its so small and cheap. Shipping would kill any good internet deal. As a bonus, you could take them your frame and they would sell you the correct part instead of hoping the online description is accurate.

Also, my first post was a guess, not a statement of fact. I know only of axles that are solid, solid with flat sides, quick release and the big fat downhill types. There might be many others but the flat sided axle seemed to be an answer to your problem, so I posted my thought. The bike shop will know for sure.