You can buy ACT practice tests in book form at the local bookstore or Amazon.
I disagree on whether taking practice test after practice test is beneficial. If a kid has the basic knowledge sought by the test stored in his brain, taking practice test after practice test can really help improve a score. It's really no different then practicing a certain play time after time; the less the kid needs to think about routine stuff, the more his focus turns to the important details. (Of course, if the basic information isn't in the kids brain in the first place, no amount of practice testing will paper over that problem.)
In the case of the ACT, the test is timed and time constraints can play a huge role in its score. Taking test after test - timed as the real one will be - gets the kid used to the rhythm and pace and ultimately creates a familiarity with the test process. This familiarity means that at testing time, the natural increase in stress (because it's "real" this time) may be reduced; and any reduction in stress makes it easier to access information already stored in his brain..
I believe that the SAT has totally revamped its format this year (or soon), so I don't know how those changes impact the advantages of taking many practice tests.
In our case, both kids hammered practice test after practice test. While I cannot say that this caused the scores, there was a correlation (correlation does not equal causation). I guess our philosophy was "leave no stone unturned" in preparing for one of the "gatekeeper" items.
As for grade inflation, it's a sort of a red herring for the top academic schools. For regular students, each college will get a copy of the kid's high school profile (showing for example, the courses offered, the average scores and GPAs of all students, socioeconomic averages, etc. of that particular school). The extreme academic colleges can then evaluate each applicant within the context of their school (so a kid attending a wealthy private school is not being compared with a kid attending a poor public school). Each kid is essentially required to have performed the best he/she could within the context of their lives. This evaluation is not necessarily true in all schools, but that's the way it goes in those top academic schools.
Standarized tests were supposed to allow schools to evauate students across the board (so students nationally could be compared). However, we all see how that ability has been warped; wealthier parents can afford better prep courses, better prep courses mean better scores, etc. So even with standardized scores, certain colleges recognize that factor and evaluate accordingly.
As for athletes, they just need to meet whatever minimum the school requires. Achieving more than the minimum can be very advantageous in schools which offer academic money - it saves athletic money and can make private schools (with their 50k+ price tags) more palatable.
I would also add this situation to ponder: an injury in the latter part of high school can completely derail athletic recruiting. Getting the best scores possible at least gives the kid his best academic alternative.