My preference is from the curry leaf tree. It forms a brush easily. They are better if you dry the twigs for a couple of days before using, however it is not a necessity.
Alternatives: rosemary twig, eucalyptus twigs, neem tree twig, licorice root, and others - I can't mention them all. A toothpick can also do the same thing if its not too brittle.
If using a toothpick, break it half. You'll get 2 brushes from one toothpick if you need. However be wise. If the toothpick is brittle, be careful not to leave tiny woodchips in the cavity.
