You finished all of your Magoosh ACT practice questions? Nice! You're clearly an ambitions, hard working Magoosher. So, where can you go from here, if you want more ACT practice tests? There are a few places.
After your Magoosh ACT practice is complete, the next place you should go is the source. What do I mean by "the source?" I mean official ACT prep from the makers of the exam itself.
Perhaps the very best source for up-to-date, fully authentic ACT practice tests is The Real ACT Prep Guide, 2016-2017. The practice materials in this book are not without some flaws (see Kristin Fracchia's review of the Real ACT Prep Guide for details). However, this is still a collection of authentic ACT practice exams, made from questions that actually appeared in recent real ACT tests.
A few other official ACT prep materials are also worth considering. The previous version of the Real ACT Prep guide (3rd edition) is admittedly somewhat outdated. But the ACT Reading, English, Math, and Science sections in this older book test the same basic skills and knowledge you'll need for the current exam. (The ACT Essay prompts in the 3rd edition are completely obsolete, though.) Then there's the content on the official ACT website. The ACT site offers a full, up-to-date ACT practice test and supplemental questions for ACT English, ACT Math, ACT Science, ACT Reading, and ACT Writing.
When it comes to other third-party ACT practice tests, the quality can be a little unpredictable. Third-party ACT prep providers are, for the most part, simply unable to make practice tests that are 100% like the real one. Still, there are some third-party ACT-practice tests that come close. Some of the best unofficial ACT practice questions out there come from Manhattan. So consider using the Manhattan 5 lb. Book of ACT Practice Questions, if you run out of official practice tests.