Getting hard to find for sure and for a 4" or a 2.5"
There seem to be a few different sizes
May want to measure yours and hope the seller has a measurement too.
I have accumulated a bunch over the years because they don't fit my needs.
Also you'll want a total length and length to where the screw hole lines up. I have seen a lot of variance over the years.
Also pay attention to the sight blade, more than likely you'll have to replace it but if you find a setup and the blade is good, consider giving more for it as it's less trouble that way. There are some springs, plungers that most don't know about in there and set up properly can run you an extra-$20-40 bucks getting the blade done right.
Look for the most complete unit you can. Stay away from rusty,crusty ones too. They're more trouble than they're worth.
You can also advertise in the Want to buy section here in the forum, several guys watch it and sell stuff to help others out. I am one of those
Just be cautious of scammers, them fools seems to be out in force sometimes.
JO