How many times did you take the free GMATprep ones? If you only took those once each, then you can retake those. The repeats are a lot less in those tests because the underlying database is large.
If you have taken them several times, then I would practice with the questions in the pool:
all-gmatprep-questions-quant-verbal-187679.htmlThen the next best would be these:
practice-paper-set-187770.htmlThe main drawback is that you won't get an actual score.
Or you could take the GMATFocus diagnostic tests.
None of these options are the same as taking a full length real CAT, but the tests from third party sources aren't any better. I wish we had some more official tests to practice with.
Dabral