It's hard finding good practice tests. I think the only good one out there is MGMAT. I was very disappointed with Kapalan's tests. I heard that Knewton is not that great, and I heard negative things about 800. But 800 is pretty cheap and you get 5 tests, so it may be worth a shot. You can also download the really old GMAC test Powerprep. You can find it on this link:all-gmat-cat-practice-tests-links-prices-reviews-77460.html
The software is really old, and so I didn't bother taking it, but you may find it helpful. It is old GMAT questions. Keep in mind there will be a bit of overlap with all the GMAC tests and the OG. I made the mistake my last time around doing a ton of CAT's and doing every problem in all the OG books. This time I have gone over the content extensively and finished all of the "advanced" chapters in the MGMAT books
. Taking practice tests is really important, but you may not need a ton. Just make sure when you take the GMAC tests you go over each question. I have been able to find explanations for every questions on Google.