There are two ways to get access to another Teacher's Lesson Plans. Both are controlled by the other teacher, so you will need to coordinate with them on gaining access to copy their Lessons.
View Groups
The first way, is to have that teacher add you to their View Group. They can either grant you View or Edit authority to their lesson plans.
Once they have added you to their View Group, you can access their Lesson Plans using the View Group Dropdown on the Calendar.
Click on the Lesson Plan you want to copy and use the Date Management icon on the Left-Side Action Menu to copy the lesson plan. The Lesson Plan will be copied to your account.
You can also use the View Group Lesson Plan Tree on the Lesson Plan Dashboard. Click the Quick Copy icon to instantly copy the Lesson Plan to your account. Pay particular attention to the Year that is automatically calculated for you, as this feature is typically used to easily copy your Lesson Plans from last year to current.
Lesson Plan Library
Alternatively, the teacher can add their Lesson Plan to the Lesson Plan Library. Once it's there, you can navigate to the Library and copy it to your account.