Some states have contracted with medical plans and you go to the providers in the plan you choose. As Sunnygirl1 suggested, try going on your state's website.
In the Triangle Area of NC, many medical offices accept Medicaid. I suppose that I would look online and maybe talk with your Medicaid Caseworker. She may be able to give you a list. Also, you might ask for referrals from friends for good doctors, then just call and ask them if they accept Medicaid. They probably list that on their website, if you have access to that.