From "https://www.pennminerals.com/collecting", it says: "MECKLEYS QUARRY - MANDATA, PA - Northumberland Co., - undoubtedly the best location in PA for celestine. Many pieces have removed from here with museum-quality crystals.  Collecting for clubs by appointment only.  Call Ted Keck 570-758-3340. There is an open house for all collectors the day after Winfield in September."

Obviously this info is subject to change.  Check for the quarry on google and resort to the phone number there if the above doesn't work.