Look into TrackDaze events on Shenendoah. They usually open the skid pad all day. A few friends once rented the Jefferson skidpad for the day in December for like $40 a person. We were lucky to have ice in the morning, then slush, then wet. Try to do skidpad events in the winter or spring due to it being hard to keep the ground wet.
Autocrosses are a good way to simulate skid pad....register for a bunch and you're likely to get a wet autocross. Maybe look into autocross school. DC Region SCCA puts on a GREAT one.