Network+ Practice Exams
ExamCompass has some free practice exams. Here is a link to the Network+ certification practice exams. I cannot vouch for how good the free practice exams are.
Fee based practice exams
Do not use brain dump websites. Brain dump websites are websites where people who have just passed the exam post the questions they remember and their answers. However, certification organizations are wise to this and they post tick information that is in the real exam to see if you got your information from a brain dump. If they catch you, you will not only fail the exam but they will also take any other certification you have away. Other certification vendors may also choose to take away your certifications if you are caught cheating.