I got some natural-bristle toothbrush style brushes from a place called The Vermont Country Store. Haven't tried them yet though. They come in hard, medium, and soft.
While not toothbrush shaped, the brushes in Griot's 015520 set can be cut down to make them much more aggressive. But note that you'll be cutting off the flagged tips, so they don't just get stiffer, they get far more aggressive overall. I sure wouldn't use them on paint after cutting them down, but the cut down ones are nice for more aggressive jobs. I've scrubbed seams/welting crevices on leather seats with them.
The issue of cutting off flagged tips might be worth considering when cutting down *any* very gentle brush....the ones I use for household antiques have flagged tips too.