I found 18-24 inch in a bedbathbeyond last week. Not much selection, but smallest I've found.

Just a rod? K-mart has them very short.

I found 12" swing arm curtain rods on countrycurtains.com. They are adjustable in length but do not appear to be sturdy enough to hold a heavier panel (I have sheers on mine). I couldn't find the fixed rods either, which is why I went with the swing arms.

We made our own from 8ft wooden poles and cut depth of standard matching brackets so that the poles would not protrude too far from the wall. We made the length of poles about 15 inches and shirred a 50 inch wide panel on each rod. Each pole has a decorative end cap.

You can make curtain rods from anything. Be creative.