This should be easy to source at a photography supply store (Samy's Camera, B&H Photo, etc.). There are not many here though - back in California they are common, here I do not remember seeing any large photography stores in PA. But you can buy it through the web - look for colored filter sheets.
Another, perhaps easier option, would be a car supply store. Buy a small amount of glass tinting mylar - it comes in several degrees of shading and are not that expensive, especially if you only need a little bit of it. I would imagine 25 of 35% tint would meet your needs. Send BJ on a mission to source you some.