My brass started from factory ammo I shot. I found once people at the range know you reload many of them will pick up their brass and give it to you. I have a bag I put my spent brass in. Often I wind up bringing home more brass then I shot. Occasionally I also wind up with some brass for calibers I don't plan to ever shoot.
The range I go to sorts used brass by caliber and bag it up. They sell it for a reasonable price. Since it is an indoor range the brass is fairly clean to start with. Some calibers they have a lot of. Others seem to be sold as fast as they can bag it. They have a lot more pistol brass then they have of rifle brass.
Starline brass is about as good as it gets.