You can search through clients or customers in the contacts window.
The search feature also has more detailed options.
On the same line as "Search options" (near the top) you will see a small arrow. If you click on this arrow you will see more options open up for your search.
This includes the ability to search just your clients, or just your contacts, or everyone (all).
The search options also includes the ability to search your active or Inactive clients, or all clients.
In the search options you can also search for a customer or client that has a tag, by typing the tag name below the word "Tags".