What does the 21.1mm measurement apply to? How did you measure it?
The quill diameter of quill stems for 1" steerers usually measures 22.x mm. Could your measurement be off 1mm? The nominal diameter refers to the outside diameter of the steerer, so try measuring this.
Unless this is a very strange bike, i would bet the steerer is 1". Quill stems for 1" steerer and 26mm clamp are probably the most common type of quill stem. For maximum stem selection, you can also get an adapter that converts 1" quill to 1 1/8" threadless. Jensonusa has these.
If i am wrong, and it is not a 1" steerer, then your options will be very limited.