Graphical designers that work on software or web pages we'll surely be happy to be able to make use of an application of the likes of Icons8. It's an icon library that can be used in any kind of project and that is compatible with different design programs such as Photoshop.
Thousands of icons available
There are thousands of icons in this application, sorted by styles. Therefore, we can find those used by Material Design, iOS or Windows, amongst others, organized by tags to make it easier to find them.
It only takes a few seconds to find any of them and they are supplied in different sizes, adapting to different developments. We can also search for similar icons in the entire collection, just in case the one we've found isn't exactly what we were looking for.
However, the truth is that to make use of them all we'll have to pay a small amount as we can't use all the sizes for free.