Check with your local pharmacies to see if any offer free prenatal vitamins.
Locally, one of our pharmacies has a contract with a pharmaceutical manufacturer to offer free prenatal vitamins in hopes of encouraging people to spend more in the grocery store, on personal care items, etc.
My OB/GYN said it was OK for me to stay on prenatals despite not being pregnant, so I figure FDA approved and free is a good deal.
Most products are registered with the FDA but not approved (those you purchase over the counter). Most you purchase online are not, and the quality of ingredients is not regulated.
I'd stick with the big name brands.
One important note, ask your pharmacist to explain how much you need of certain vitamins that you're not receiving through your diet. Calcium, for example, comes in pills as large as 1000mg, but your body can only process 500mg at a time. You basically urinate out 1/2 of the pill needlessly.