If you only use a few icons a year and you do not need to choose only flawless then Ventura, a non-commercial business, offers several options for free or cheap icons and images.
At IconFinder.com you can filter the content so that only free options come out. For this, select "No link back". The luxury of choice is not really great, though it is not so scarce that you can not choose anything
Note: If you do not want to spend money on the icon, check that it is marked "Free for commercial use" or "No link Back"
If you use only a few icons however you want to choose as good options as possible.
Paid icons on IconFinder.com cost about $ 6 ($ 2). Additionally, the web page has a very handy icon editing tool to change the color, add or remove elements.
Note: At IconFinder.com you can also filter by icon type, such as black (glyph), outline, full, etc.
If you use more than 20 icons a year and their quality plays a crucial role.
In such a situation TheNounProject.com is a good choice. For only $ 40 you can download as many icons as you want, as many times as you want, so you do not need an attribution. Additionally, you can change the color, though it can not be further edited.
Note: If you need an icon for more abstract purposes then you can Google it: "your abstract concept icon." This will allow you to take note of what others are using. Then use a more specific word to search.
The most extensive choice
The largest selection of free websites is on the following websites: Pixabay & Pexels. Both were purchased by Canva recently. You can browse Canva.com/photos and filter for free to see the most affordable options
Also interesting options are:
Note: If you use a lot of stock images and you have no problem paying, Canva also has an unlimited download set for only $ 13 per month. If you only need a few a year, standard images on iStock.com cost around $ 11.
The problem of belonging
Many icons and images are free at first glance, but you will soon find that you need to link to the author. If you decide not to do this, you may not be able to understand it, but being natural does not mean it is not bad behavior. In such a case, it is better to download images and icons whose authors do not request such a thing.