Try e-gunparts.com, gun shows, gun shops, want ads.
Numrich (Gun Parts Copr.) has some Iver Johnson parts.
gunpartscorp.com has the best collection of Iver Johnson parts
These were made by Iver Johnson company. Try searching gunpartscorp.com under Iver Johnson,
You might try http://www.e-gunparts.com.
I would start my search at Numrich gun parts corp.
Try Numrich Gun Parts at the related link.
Possibly Numrich Gun Parts of NY.www.e-gunparts.com
you can get parts for most guns at Numrich gun parts corp. they are on line, and send parts out very fast
On line auctions, gun shops, gun shows, pawn shops.
the 'bluebook'
Online! Search Iver Johnson information; There are forums that have experts or links to answer any question you have. I have yet to find a single source for everything Iver Johnson...
Start with Gun Parts Corp and then start looking hard. Old gun parts are getting hard to find.