Go to free or reduced cost clinics and ask. Because I was under the income threshold, I was able to get all of my labwork done at my local university clinic for $1 (I was not a student, the clinic was open to non-students as well). But seriously, check out schools, any local low-income clinics. I might even try a free sexual health clinic that does STD screening. Get all of your receipts. (They used to reimburse some of the cost).
For the dentist, I went to one on their list. He was great, very supportive of the Peace Corps, understood that I had no money and tried to give me the cheapest options possible (while the x-rays and exams were free, he did charge for the 1 filling PC insisted that I get).