Superior quality brushes make all the difference for line work and other effects when face painting. Don't buy the cheapest brushes you can find as it will show in your work.
Any good mid priced watercolour brush should work well for face painting and doesn't have to be bought from a face painting shop.
If your brush has got bent and out of shape try the following:
Put some boiling water in a mug and then pop your brush in. Run the brush alongside the mug as you raise it out of the water to keep the shape. Dry standing upright to retain the brush shape.
Apparently hair gel also helps re-shape brushes using the above method.