How can you get heavy metal poisoning?

Industry. Known exposures are usually in industry, and depending where you are, industrial medicine / hygiene may be imperfect and allow unfortunate things to happen. Lead from paint / plumbing in old homes is serious. Arsenic and less often thallium are homicidal poisons; I've run into both. There are crooked labs that will overdiagnose heavy metal poisoning. Good luck sorting it out.