You might be able to use one from an original 1911- the narrowest version of the tenon is the same size as the 1903, but the sight narrows to a pin at it's base on the 1903, so some file work is inevitable, I think.
Worst case, you could make some out of flat stock pretty easily (if you're into stuff like that) and stake it in place.
Another resourse might be Marstar (sp?) in Canada.