You might also check out
this free test prep site, if you are worried that independent study isn't the way to go for these tests.
My personal view is that knowing the underlying material that is covered on the tests and getting comfortable with the format of taking the tests (which can be accomplished by running through free or cheap practice tests) is really all that is needed to do well. A lot of the highly-rated test prep courses really just help remediate bad high school teaching and provide some basic test-taking tips, such as answering all the questions you know right away first, before spending time trying to figure out ones that you find more difficult.
