Rob once gave me a tip to look out for broken fork lift arms as a good source of heat treatable steel.
For aluminum in chunks I generally revert to casting, unless someone gives me some scrap. That can sometimes be had if you have friends in a machine shop or operation attached to another business. They usually have a scrap bin for mistakes, or beaat up stock, etc.
For casting aluminum--ium--whatever scrap, cruise around town on trash pickup day -- aluminum--ium-whatever can be found in various car parts, like some bell housings, even some blocks, lawn mowers, lawn chairs, discarded window frames, etc.