Donthrash,
There is no serial # information available for the M-1865 Spencer; your model was produced to late for use in the Civil War; it nay have been sold to the civilian market and made its way West - or may have been issues to a cavalry unit fighting Indians.
Do a Google search for Spencer parts; Original Spencer Parts.
SSFirearms and also Lodgewood have parts; also try eBay too!
Check the SORI Thread on the first page of SSS for lots of information on parts and repairs.
Two Flints