Planned Parenthood will probably be able to help you. They do a sliding scale fee based on your income and work on the policy of "we would rather you have the care you need, even if you can't pay." At least the one in my area does and the ladies there are wonderful. I'm hoping the doctor there gets into regular practice as an OB before I ever have another child, she's great. They can help you with most aspects of women's reproductive health.
With a regular obgyn, you can certainly explain your situation and they may be willing to help. The worst they can do is say no, right? Many times you can talk to the business office and they will give you discounts for paying all at once (even if its not at the time of the visit) or will allow you to make payments. You can even make sure the doctor knows you are not covered by insurance. I let my doctor know at my post-natal check up and she said she was more than willing to work with me. The business office had said the bill might be up to 300 dollars, I got the bill and it was $50.00.
You never know until you ask and its worth asking! Most doctors do care and will help you out.