I use several sources. One is eBay, two is coin dealers/antique dealers, three is recycle dump. Another good one is real live auction houses for high value rarer stuff...
I recycle any copper or pipes, scrap pieces of aluminum, lead sinkers or shotgun slugs that are not old or valuable. I collect about a 5 gal bucket full every season... The recycle yards pay me about $100- $150 a season for that type of stuff I dig. We don't do this to get rich, its fun and sometimes money is a part of it. I'm lucky to earn $1.50 an hour metal detecting, haha..