You can visit https://iconscout.com/unicons/explore/line to download hundreds of icons for free. It is highly recommended to use svg icons provided by this website, you can use other svg icons but you may encounter issues when displaying the icon.
Here are the steps to download category icons:
1. Create a free IconScout Account
In order to download icons from IconScout, you need to create a free account. Head to https://iconscout.com/ and on the top right click on Signup and signup for a free account.
2. Search for Icons
Go to https://iconscout.com/unicons/explore/line and search for an icon, example
Tree to be used for parks:
3. Download the Icon
Click on the icon you want to download and on the right side, click on
SVG button under Download.
You don’t have to set the icon colour, the icon colour can be changed via css code.
3. Upload Custom Icon
Nearby Places > Categories and add a new category (or edit a category you created before). In the Category Icon, upload the SVG image you just downloaded and add new category or update it.