"...looking for earth"

A thing to remember when you're trying to figure out how to fix something like this is that earth is not literally earth, and may be a thing that is not in fact even connected in any way to the earth or electrical ground. What you want to think about instead is that the spark jumps between two things.

What you're looking for is a way to connect together the two things that the spark jumps to and from. A connection from the roller to the brackets will probably solve the problem. These sparks are VERY low in current, so even the thinnest wire will keep a charge from accumulating.

A capacitor would store the charge, allowing for greater current when it discharges, so that, by itself, would just turn a small spark into a show. Lights are basically resistors of some sort, and would work, but since the current is so low, a wire will work.