I've just gotten the cheap short, stiff bristle brush at the grocery store....kind of hard on the nail beds but great for scrubbing the dermis. Used, dishwashed toothbrushes are free and plentiful for softer bristles.
I just use the dollar store specials. While it may seem counterintuitive, I've found the finer bristles actually do a more thorough job. The coarse bristles are great for quick removal of the heavy grunge, then the fine for the finish.
I have a couple that came from counter area of AZ or O'Reilly's. They are those ones they try to sell you as you're checking out. One has been in the shower at home for a few years and I have another in the garage bathroom.