Amazon. There are even strips. Now can we trust the manufacturer of the tests themselves given amazons sometimes shitty vendor quality? Who knows.

I have a gas engineer test my appliances every year. Last year he also fitted a CO alarm and he said he would test it every year. I asked if he had a CO aerosol to test the alarm. No, he said, I just press the button. Who knows if the alarm actually detects CO? Anyway, if you want to test your lead test, introduce it to some lead.