Top photo: Daiso brush in packaging.
Bottom photo: Soft 4" Daiso brush to the left, 3" hardware store chip brush on the right.
Shaving brushes should work too, but they are significantly more expensive than even a super soft paint brush from Home Depot or your favorite art supply shop. And if you actually shave with it, you should probably clean it carefully to get all of the soap residue out before applying to synths.