How do you find out about pro bono dental care for people with no income?

I am a former cancer patient that recieved radiation causing severe tooth decay
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See if there's a dental school in your area. They always need people to practice on (and all the students are well supervised). I don't know if they'd do it for free, but you might could work something out. They are generally half (or less) than you'd pay at a regular dentist's office.
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