You'd do better to search for "Boat Ramps" in the US. We generally do not refer to them as slipways.
In general (and this is very general, so don't rely on it too much), boat ramps that are built and maintained through public money (City or State funding) are free to use, but there will be a charge for parking. Ramps built or maintained through other than public money will usually charge a use fee, often a per use (i.e. you pay to launch and pay again to recover.)
Don't know how detailed or up-to-date this is, but you could search this:
Boating & Fishing Ramp Locator - BoatUS