Marc Brunet offers a popular Starter Brush Pack for free, his Advanced Painter's PS Brushes
The "Advanced" set is a separate, more comprehensive collection used for professional-level speed painting and complex textures. marc brunet advanced brushes free
If you are a student, you can access a lot of the advanced brushes through Marc’s Art School for Digital Artists program (which is often free during the first week of the month). While the course costs money, the exercise files—including the brushes—are downloadable during the trial. Marc Brunet offers a popular Starter Brush Pack