Delrin balls. McMaster/Carr will have them if you know the size. But instead of browsing the Harken site or going looking for the right balls on McMaster, call Harken directly if it is a Harken traveller car you have. Their customer support is usually very good.
In addition to the balls you also want a piece of track with one end sawn off at an angle (not a right angle but about 60degrees). That is the easiest way to re-fill the car with balls. Dont put in too many balls either, that will make the car stick! They need room to move.

Dont ask how I know all this