as of today, there is no "official" source of practice tests. Programs, software and practice tests developed by private companies are the only sources of practice. Some of these are similar to the actual DAT, some are too easy, and some are too hard. The best way to prepare for the DAT is to study hard. Except for PAT, there no real "tricks" to any of the questions - either you know how to do them or you don't. And the only way to do them is the study really hard.