Numrich Arms Gun Parts Corp. West Hurley NY. is a good place to start
Try e-gunparts.com
Numrich gun parts
Check old Taurus catalogs
By it's model name & serial number.
By model and serial number. Go to proofhouse.com
Try e-gunparts.com
On an older revolver, you are not likely to find a model number, since they did not use model numbers. If you remove the grips, you should find the serial number stamped in the frame under the grips.
Crosman offers a free service to locate repair shops that repair older model Crosman airguns. see the link below.
You can find the assembly diagram for the Iver Johnson model 1900 revolver in several places, including firearm manuals, online firearm forums, or websites specializing in gun parts and restoration. Additionally, resources like the Blue Book of Gun Values or specific collector's guides may also provide detailed diagrams. Online platforms like eBay or Numrich Gun Parts Corporation might offer parts and manuals for purchase as well.
You will have to call Rossi to find out.
You will have to call S&W to find out.
YOu will have to call Colt to find out.