Twin Knobs like Zilpo has it's own ramp inside the camp ground you can buy a yearly pass at the camp ground and use them if your not camping there. Passes for all other ramps can be bought at any bait shop or at the forestry office near the dam. Leatherwood, Bangor, and Black Water ramps do not require a pass.
All boat ramps are open to the public as far as I know. The launch pass is required and can be bought at almost any store or baitshop near by. There is a BP at the Farmers exit off I 64 that sells them.