Hi AvMo,
The most realistic CATs available are the 6 from GMAC, but retaking a CAT that you've already taken is NOT a realistic way to assess your skills - since you will likely see some 'repeat' questions that you have already faced. Unfortunately, seeing even a couple of repeats can 'throw off' the Scoring Algorithm and impact your pacing, energy levels, fatigue, etc. (meaning that they would likely all appear to be better than they actually are). Assuming that your goal when taking a CAT is to receive a realistic score assessment that will help to define your skills, strengths and weaknesses, you should only take each of those CATs ONCE (and take it in a fashion that matches-up with the parameters that you will face when you take the Official GMAT).
GMAT assassins aren't born, they're made,
Rich