#1). Hate to sound obvious or pendantic, but try Llama. They been around a long time and might still have some parts for anything.
#2). Brownells.com takes questions and is usually on top of the business. If they don't got it, they might know who does.
#3). Find a hard copy of Shotgun News--shotgunnews.com. Peoria IL now I guess. Used to be Columbus NE. In there there are advertisers who do parts. Some have good web sites and some don't. Navy Arms comes to mind. SARCO, NJ. Tom Forrest. I guess Springfield sporters went under. Don't recall the name of the man specializing in "sat. night special" parts, but might have some Llama. Early guns were, reportedly, "soft." All else fails, you can run an ad for what you need. I suppose Gun List might be as useful, but I got into SSN long before there was a gun list...
#4). Last, but not least, what can you substitute. Many, many copies of the 1911. Will parts from another gun "fit?" I guess Howa made their receivers to use Remington 700 bases... You just have to look/ask... #2 is good for that. luck.