Quoted:
You can order them through most local ACE hardware stores. I don't know about being magnetic but I guess you could magnetize them. Size is Cap Screw, 1/4 x 28 x 1 inch. I bought two boxes of 25 for about $11 a couple years ago if I recall the price correctly but don't hold me to it.
I think the OP misspoke about the magnetic part.
I've never seen a magnetized screw for an PG, however all the black oxide ones are ferrous and attracted to a magnet.
Good quality stainless steel screws like most of the slotted ones used in LPKs are not ferrous and therefore aren't able to be magnetized.
That being said, a lot of 'stainless-steel' that is starting to show up on the market is low grade crap from Asia that does
demonstrate ferrous properties such as the ability to be magnetized and will corrode easier than it should.
Gonna buy stainless fasteners at your local hardware store?
Take a magnet...