Kedar signifies the whole East, and the isles of Chittim (Isaiah 23:12 note) the West. If then you traverse all lands from west to east, it will be impossible to find any nation guilty of such apostasy as that committed by Israel.
Related Commentaries of Jeremiah 2:10
Jeremiah 2:10
10 For pass overc the isles of Chittim, and see; and send unto Kedar, and consider diligently, and see if there be such a thing.