eBay is a good source, but it sometimes takes a little work. I've found some obsolete aperture sight parts for very reasonable prices.
That's usually used on the Winchester Self Loader center fire rifles.
I have seen them mounted on the WinchesterModel 03 and 63 22rimfires also.
Winchester 95 perhaps.
Ken Bean was always a great source. Sad to see him pass on.
I did not know his business had been sold to another person. That is good news.
Other than that,,Ebay seems to be the place to look.
Gun shows,,I always look but pickin's is sparce.