Where can I find unit icons?

Where can I find them? By unit icon I mean the art for the unit that appears when you search for it in the civilopedia.

As in Vanilla? Google it, they all come up with an easy search.

You could Google them, yes, but the quality may be hit-and-miss.

If you want the original files:
They're .dds textures packaged within the game's .fpk files (which are located in the resource subfolder of the game's installation folder). You can extract them with the Nexus script (part of the SDK) or use Dragon UnPACKer.

You'll need this DDS unpacker tool to convert any of the files that have an accompanying -index.dds into readable .png files.