https://www.amazon.com/Winsor-Newton-Kolinsky-Sable-Watercolor/dp/B0013E68T4/

Size 1 (I have smaller brushes but I only break them out for 6mm miniatures)

Representative examples of what I use:

Synthetic Brush Pack: https://www.amazon.com/gp/product/B002KQYC74/

Winsor & Newton Series 7: https://www.amazon.com/Winsor-Newton-Kolinsky-Sable-Watercolor/dp/B0013E68T4/ (i only have this one brush, I don't use it except for layering)