I use the Colgate Wisp disposable toothbrushes. They have a little hard toothpast lump in the middle but I pop it out. It's just the right size for mine. They make toothpaste for dogs that comes in all kinds of yummy flavors like beef and chicken. Here's a link for the toothbrush I'm talking about. You can pick it up at any drug store though.
Amazon.com: Colgate Wisp Peppermint, 16-Count...