I'm not knocking how anyone else does it, nor am I saying my way is better, but I don't fuss too much over the chemical composition of my mongrel "alloy."
I scrounge whatever the heck I can find. My base is 1990s and older wheel weights, but I can and do "soften it up" by going 50/50 with that and "pure" lead, like old foundry ingots, roof flashing, plumbing bits, etc. I have some stuff that is really "hard," which clatters and "tings" when I drop an ingot. I've found that if I cast that at a higher temp (like 800F) is casts just fine and shoots fine.
I have shot up to the mid 1800s (fps) using aluminum gas-checked/tumble-lubed bullets, very accurately and without leading, from the 222, 357 Mag and 30/30, in rifles using straight wheel weights - air cooled. I know some folks who would swear this isn't possible, but then I cast and shot much more successfully before being "educated" by such wise ones. I have slowly come back 'round to my old ways of casting with what I have, shooting it and determining if it's OK. It's almost always OK.
I may just be a lucky fool, but I use what I have and try to shoot as soft a bullet as I can get away with for everything. In other words, take whatever you can get and experiment. I don't know another single source outside of THIS forum for truly good information on the topic, so I agree with pretty much everything everyone else has said so far, but it doesn't change my tack o the topic of alloys - for ME in MY guns.
Wait, OK, there's one other source I do trust unquestionably, even though I don't do everything the way he does, and that's Glen Fryxell, on LASC, who is a member here also, so he's within that umbrella of trust for solid and objective information.
Take whatever "lead" you can get your hands on and try it - experiment. If you get lead that's not ideal or can't be blended into something useful, you can always trade it to someone making sinkers or in need of ballast for whatever. I know a guy who poured large coffee cans full of "lead" to make wheel weights for a garden tractor. He didn't need good ballistic alloy for that.