Hi veeralshah,
When it comes to CATs that you can purchase, there are a variety of respectable CATs available. While none of them is an exact match for the Official GMAT, they are all reasonably close. There are sets of CATs available from Kaplan,
MGMAT and Veritas as well as the 2 additional CATs that you can purchase from GMAC.
Keep in mind that a CAT result is only as realistic as YOU make the CAT-taking experience. Thus, YOU have to take the CAT in a realistic and test-like fashion, including taking the ENTIRE CAT (with the Essay and IR sections), at the same time of day as your Official GMAT, away from your home, in a test-like environment and NOT do anything unrealistic (pausing the CAT, listening to music, skipping sections, etc.).
While it might be tempting to take lots of CATs, doing so is NOT what will make you a better Test Taker. A CAT is just a 'measuring device'; actual improvement comes from study, practice problems and quizzes, review, etc. The CAT is a way to measure your progress.
GMAT assassins aren't born, they're made,
Rich