There are many places where you can find Mathematica Slideshows, created not only by users but also Wolfram Research. For example, these free training courses
http://www.wolfram.com/training/courses
very often have a notebook of the course attached to the top right corner of the video:
http://www.wolfram.com/training/videos/DAT021
Another place is Wolfram Technology Conference:
https://www.wolfram.com/events/technology-conference/2016/presentations/#wednesday
Here on Wolfram Community people also often attach presentations, for example some posts here (see attachments at the end of the posts):
http://community.wolfram.com/web/ihcomm