Where I can find standard Ubuntu icons?

I need icons which is near the name of program in left-top corner (for nautilus and text editor). And icons for text file and catalog in nautilus. It is for my site.

You want gtk3-icon-browser

You will probably have to install gtk-3-examples.

sudo apt install gtk-3-examples

As you stated, this will only give you "standard" icons. There won't be any vendor specific icons.

This application only gives you the name and available sizes of the icon so you can search it (like with the methods in the other answers. ie /usr/share/icons)... The only extra benefit is that it provides somewhat of a categorized database that you can search.

Not sure what you are looking for but you can use the find command in a terminal:

[sudo] find directory -name '*.png'

where

• sudo should be omitted as long as not necessary
• directory is where you want to recursively start the search.

This is of course subject to all variations you need. To search the directory mentionned by user535733:

find /usr/share/icons -name '*.png'

To search your whole system:

sudo find / -name '*.png'

Use with care and help yourself a coffee while it runs.

In Ubuntu, system icons are available in /usr/share/icons directory. The standard Ubuntu icon theme is called Yaru and it is installed by the package yaru-theme-icon. This package puts the Yaru icons in /usr/share/icons/Yaru

Another way to get these icons (useful if you don't have access to an Ubuntu system) consists in going to the GitHub page of the Yaru project. Clicking the green button labeled Code and selecting Download Zip, you can download the archive, then extract it to access to the icon directory.

Generally speaking, another directoy where icons may be placed are ${HOME}/.icons or in ${HOME}/.local/share/icons. You may find there all the icons pack that are manually installed by the user not by apt/synaptic.