The best option is to look on Ebay where many people are selling parts for products they no longer use. You will most likely be able to get a pretty good price on one and can throw away anything else that comes with the auction that you don’t need.
There are also battery stores that specialize in hard to find batteries. It’s all they sell. By finding a local one of these, including Batteries Plus, either in store or on line, you should find what you need.
These stores are a little more expensive, but if you really like your Benchtop Pro, it may be worth it to you.
Ultimately, if you strike out at both of these, you may have no choice but to junk your Benchtop Pro and buy a more modern version. You can go into Home Depot, Lowes or any other home improvement store and let them know what you have. They’ll either be able to point you in the direction of a place where you can procure the battery (unlikely) or show you a new model that has batteries that are much more accessible.
Finding a unit of anything where it uses common batteries is much easier than ones specifically for one piece of equipment. There are many brands out there that allow you to use one battery and charger for all of their lines of equipment, making it easy to trade between appliances without spending a fortune.
A new model may also end up being cheaper than going on a wild hunt for a battery that is no longer being manufactured by the company that originally made it.