Cold rolled steel, 1/8 or 3/32. Scrap usually. Discarded angle iron and a hacksaw has provided me with lots of material over the years. I just case harden it when done. Casenite is nice but I guess they don't sell it anymore. I've got enough to last me, but there's some other surface hardening compound(s) around now.
If I need something thicker like the bolster area on a trigger plate, I usually build up that portion by brazing on a piece to the strap. Just easier than cutting a thick piece down and leaving the bolster in place,,at least to me.
If you braze it, you can still case harden it w/o it falling apart. Silver (hard) solder will let go under case hardening heat. But I've used it too and not bothered to case harden the plate at all,,works fine.
Annealing old flat files and cutting them up is a good scrap source of tool steel. I used to throw them into the wood stove and let them cool w/the fire to soften them.
I don't burn wood anymore so I heat them dull red w/a torch. Then bury them in a metal office size trash can I kept that's 3/4 full of wood ashes. Works for slow cool annealing of any part(s). Bury it in the dense ash and comeback tomorrow and pull it out. It'll still be warm sometimes.
You have to be able to harden and then draw the temper back just right on parts made of the high carbon steel.
Casehardening low carbon steel is probably easier for most.\
A couple feet each of 1/16, 1/8 and 3/32 drillrod provides a lot of pins but you can also scrounge different dia rod from scraped out junk. Case harden the ones made from low carbon steel if you feel better about it.
Buy some 1095 or other spring steel to make springs and avoid the heavy labor input on an unkn piece of steel.
Unless you have scrap that you know will harden/temper and hold up in use,,,it's awfully frustrating to put hours of filing and fitting into a spring only to find the steel itself doesn't harden and temper by any home shop method.
Short of that, I use pieces of old broken (main)springs sometimes if I can find a piece large enough. I hate throwing anything away so there's always a junk box to go thru to see if I have something I can 'fit'.
By the time you get through, you often ask yourself why you didn't just start from scratch.
But that's just the way we are I guess.