The most reliable way to get these is through the publisher's official teaching and learning materials:
If you want to supplement your learning beyond the Menschen series, the internet is full of excellent free materials for A1.2 learners: menschen kursbuch a12 answers free
Simply copying answers into your book gives you a false sense of security. When it comes time to take a Goethe-Zertifikat A1, Telc, or ÖSD exam, you won't have the key. Use the "Three-Step Method" The most reliable way to get these is