Just about anywhere...Walmart, Home Depot, Office supply stores, etc. Dollar stores probably have it too, albeit a no-name brand that may or may not work as well.
And yes, it is reuseable. After use, I pull it back off of the toothpicks or popsicle sticks and knead it to move the dried paint around, keeping it tacky. After many uses, I toss it & use fresh, it's cheap