Try e-gunparts.com, gun shows, gun shops, want ads.
Ivers Johnson has an Owl head on the top of the grip
You have to look it up in a book like "2008 Standard Catalog of Firearms" or a similar book. There is a large section on Ivers Johnson firearms.
Numrich gun parts.
You will have to check features against old catalogs, magazines, gun books,etc.. SN's were not required before 1968
Numrich (Gun Parts Copr.) has some Iver Johnson parts.
Start with Gun Parts Corp and then start looking hard. Old gun parts are getting hard to find.
Try e-gunparts.com, gun shows, gun shops, want ads.
On line auctions, gun shops, gun shows, pawn shops.
Try Numrich Gun Parts at the related link.
I would start my search at Numrich gun parts corp.
Scott Ivers's birth name is Scott Roger Ivers.
Look up Numrich at I think: gun parts inc west hurley NY