There are so many good resources online. The best include:
MGMAT tests and questions banks,
GMATClub Tests, and Jeff Sackmann's problem sets. However, most of these require a great deal of concentration, so you might not notice someone sneaking up on you. You can't blow your cover, as then they will be all over you.
And I agree with resh that you wouldn't want to use the more challenging questions. It's better to do those in a quiet, calm environment with no distractions. That way you will have time to complete and review the problem. This leads me to recommend the
MGMAT question banks (not the CAT's) and, as always, the questions found in the GMATClub forums for the workplace.