Trumpet/Cornet Exam Books
Exams include sight-reading tests and scales, and you can find books to help you with those elements here.
You will usually need to complete an aural test as part of your practical exam, too. You can find books on the aural tests here:
Aural Exam Books
You can find the books with the pieces from the exam syllabus here:
Trumpet & Cornet Exam Syllabus Books