Gun Parts Corporation carries almost all the parts for the 620.
http://www.e-gunparts.com/
Gun Parts also sells an exploded view of the Stevens 520-620 series guns for $1.00:
http://www.e-gunparts.com/schematics.asp
The Stevens 520 and 620 were used by the Military during WWII, so you will probably be able to find a reprint US Army ordnance repair manual for it.