You can find a variety of paints and brushes at any art or craft store.
Choose paint colors that your child will enjoy, and pick brushes with different shapes and sizes to create different textures.
Fingerpaint is also a good investment, it's important to choose one that is safe for your child to use.
Water-based paints are usually the best option for young children.
Getting a smock for messy paint projects is a good idea.