You can download the free official practice tests from
www.mba.com. They're called GMATPrep. Those are a must-do and the most accurate.
A few sites have free tests that you take online. I'm not sure of all of them, so hopefully others can post links. The Princeton Review has one:
https://www.princetonreview.com/business ... ?uidbadge=I believe Kaplan has a free one too, but in my experience Kaplan isn't really reflective of the real test.
For the most part, you have to buy tests. It's too easy to pirate them, so most companies don't make electronic versions available. I also got PR's book, Cracking the GMAT, which has a DVD with access to four more tests on their site. So far those tests have been pretty good for me.
Hope this helps as a start.