Here is a very useful collection. I am sure you have noticed the PowerPoint Icons – 450+ beautifully created SVG graphics. The problem is that you need to select and download icons on demand. Two problems. Firstly, it requires an internet connection and second problem is the need for inserting the same icon again and again.
I solved both problems by manually downloading each and every icon available today (7 Nov 2017). You can download this presentation (5 mb) and get offline access to all icons. Remember, SVG format allows these to be scaled without any loss of resolution. SVG support is available only with Office 2016 and upwards and of course Office Pro Plus.