I buy colored Hippity Hop balls off of Amazon that match my boat. They come in all different sizes and are the exact same thing without the advertising and many of them come with a little hand pump to pump them up. I use Velcro straps to loop through the handle and the cleat to keep them in place when tying up to another boat or dock.

example here:

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1