The short answer is:
Many universities provide access to the IRC via library proxy. If you are a teaching assistant or adjunct faculty, log into the using your school credentials.
A key feature of the Mankiw Macroeconomics 10th Edition lecture slides is the integration of "Active Learning" discussion questions designed to boost student engagement. Slideshare Notable PPT Features: Balancing Theories
You can often find the first 5 chapters of the for free via university "sample chapters." However, chapters 20–22 (the crucial monetary/fiscal policy section) are usually locked behind a paywall or instructor login.