The Sterling semi-autos were only imported into the US for a brief time in the 1980's. Import stopped in 1990.
The importer was Cassi Inc, of Colorado Springs Col.
The model was the Sterling Mk 6.
Gun Parts Corporation carry SMG parts and schematics:
http://www.e-gunparts.com/model.asp?idDept=240
Possibly they MAY have parts, unlisted. Contact them.