Congrats on the JNCIA and moving on to JNCIS.
Obviously the topic list is your starting point here which also lists the three official formal classes that cover the topics in this exam.
https://www.juniper.net/us/en/training/certification/certification-tracks/sp-routing-switching-track?tab=jncis-sp
learning path
https://learningportal.juniper.net/juniper/user_activity_info.aspx?id=5357
Ideally, your employer could treat you to those formal organized courses.
There are also two upcoming free Open Learning webinars that cover prepping for JNCIS-SP you can sign up for here where all the prep webinars are listed.
https://learningportal.juniper.net/juniper/user_activity_info.aspx?id=11478
Use the topic list to cover the information self study. The Junos Genius free program does have a sample test and a number of short free modules on some of these topics.
https://cloud.contentraven.com/junosgenius/login