On the most recent builds you should find both belts and chains. They are currently identical mechanically.
Where can you find belts?
In the Robot Workshop, look under the "Motion" tab -> Transmissions. You should find a variety of options to play with.
Once you place a pulley, select it and drag one of the arrows to pull out a belt or chain in that direction. You can make belts and chain as long as you need, and adjust the gear ratio by changing the size of the pulleys/gears.