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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.