I just ordered the 2M-136-6 (GT2-136mm) belt from Amazon for $7.99: https://www.amazon.com/gp/product/B07VZ5L2R2/ This is to get me up and running.
Since I was thinking about it I ordered 3 spares from China, $0.40 each plus $4.00 shipping: https://www.robotdigg.com/product/805/132mm,-134mm,-136mm-2GT-endless-belt
Be sure to specify the 136-2gt-6 belt (136mm long, 2mm tooth spacing, 6mm wide), or whatever size you need.