Textbook solution manuals are basically past papers together with their marking schemes to help you prepare for your examinations. Therefore, it would indeed be helpful for you to find websites that carry relevant textbook solution manuals that you could obtain and then work with towards your preparation for an important test or examination.
They may not be entirely necessary but if you are looking to test your knowledge and your ability to answer questions of all types relevant to the textbooks that you might be using given that sometimes questions that appear in your exams may be tricky then you would obviously find it important to have access to such textbook solution manuals to help you develop your ability to solve questions effectively.
The only thing I recommend you to do is to look for websites offering textbook solution manuals that are actually reliable and trustworthy given that you will be basing your decisions upon the solution manuals that you come across and this would have a direct impact upon your performance in your various examinations. It becomes even more important to find a website that you can trust for your textbook solution manuals when you expect to pay for the privilege of gaining access to relevant textbook solution manuals.