The band should not need to be replaced very often (or at all) if everything is dialed in for optimum performance. See: Step- by-Step set up instructions: https://swiftpaws.my.canva.site/pounce-get-started
We do not sell the rubber band separately, but if you need to replace it in the future, look for a 6" (circumference) rubber silicone band wide enough to fill the space on the pulley wheel. It will measure 3" when folded in half.
Try a google search typing in "silicone rubber bands 6 inch" - this should generate many results to choose from.
Here is a link to just one example - https://grifiti.com/products/6-inch-standard-bands-5pk?variant=15658766401645
This is not a recommendation - there are so many companies and options (color, quantities, prices) available and SwiftPaws does not necessarily have experience with or information on the company or their product. - please use whatever you find that works the best for you, from a source you trust.