The Orange Borg - Resistance is Futile! You will be Assimilated !
There's a firm which supplies hardware stores with racks of divided drawers containing unusual fasteners. I can't remember the name right now, but you've seen them. That's where to look for 1/2-20 NF.
Another place you might find a 1/2-20 thread is the head bolt or manifold bolt of an older American car.
With apologies to Star Trek, The Next Generation, there's a huge orange Borg travelling around, assimilating all the small hardware stores.
Just like the TNG Borg, "Resistance is Futile! You will be Assimilated!"
Newcomers to this board may not have noticed the "Sticky Thread" up at the top of the forum, where we try to list the few remaining great hardware emporiums.
Cherish your small stores. Buy whatever you can from them, even seasonal things that are cheaper at The Big Box. (Just knocking off the lawn care sales is enough to put some small shops in the red.)
Oh, sure! You can buy all your stuff over the internet, maybe even from a supplier which is heavily into robotics to pick, pack, and ship! If you do, don't be surprised that the main street of your town becomes a series of empty storefronts, and your kids can't find summer jobs.
I grew up in a suburban town of about 18,000 population. There were 5 hardware and paint stores plus a lumberyard within walking distance of my boyhood home. Each one had a unique stock which reflected the experience and opinions of their proprietors. If one of them did not have what I wanted, then likely one of the others would! That town is down to one paint store, one hardware store, and the lumberyard. Since the Orange Borg has opened up in the two adjacent towns, I fear for their future.
On second edit: If you want to stock up, all sorts of odd lots of fasteners can be found at Flea Markets and auctions. Be judicious!