The current official book for the SAT, The Official SAT Study Guide, has eight official tests—tests also available online from CollegeBoard. The current official book for the ACT, The Official ACT Prep Guide, has seven official tests.
The official books have some useful information on strategies and prep. You can also search the official websites, collegeboard.org and act.org, for possible additional practice materials.
Khan Academy has free, quality help for the SAT. If you have taken an official (P)SAT, link your CollegeBoard account with Khan Academy. Khan will analyze your test(s) and provide ongoing practice where you have the most room for improvement. If you have not taken an official (P)SAT, take diagnostic tests on Khan that will also lead to ongoing targeted practice.
Khan is not affiliated with ACT and has no ACT-specific materials, but Khan’s Grammar review section, located under Reading and Language Arts, can help on the ACT English section. For Math, search for specific ACT Math topics on Khan Academy—you’ll frequently find helpful material.