I bought a Stevens 520.

I need an inertia block spring.

Anybody have one to sell or borrow to get measurements of it to have one made?

I know...VERY long shot.

did you try Numrich Gun Parts?  They carry many parts for old guns.
https://www.gunpartscorp.com

Jack First Parts.

Yup, checked ‘em both....both out of stock.

Had time at work today and looked at 6 or 8 pages of a google search. Either out of stock or not listed as available.

Got a heck of a deal on the gun though...$100

Been buying a lot of older guns lately.  It can turn into a rat hole very quickly.  I needed a firing pin for a Beretta 418.  Eventually found one.  $90.  Needed grips for a Colt Commando (the WW2 revolver).  A surprisingly difficult item to find and expensive when you do.

Try Ahlman's in Minnesota. They've got a whole room full of junk guns-stuff they've considered unrepairable and are either parting out or selling as-is to someone who has more time to look for parts.

ETA-You could also try calling John Sieh in Clear Lake(IA).  Pretty sure his number is still listed in the phone book. He used to be the go-to guy for properly done, low cost gunsmithing. If you didn't need it fast he was the one to go to-it would be done right.  Doesn't do the work anymore(age caught up to him) but he's still in the hobby. He has fishing tackle boxes full of small parts-or he did the last time I talked to him.

Well aware of that. Bought a Remington pre-model 10 recently. Being 112 years old, it's got a lot of wear and misfeeds. Got that working, but now something is stuck in the action, keeping the bolt from closing enough to lock. I needed a project anyway.

All good leads that I will follow up on.

That’s what this was...just a project.

Turning into an obsession.