example source code that displays the letters Ç, Ğ, İ, Ö, Ü, Ş using HTML character codes:

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>HTML Character Codes</title>
</head>
<body>
<h1>HTML Character Codes for Turkish Letters</h1>
<p>Here are some examples of Turkish letters:</p>
<ul>
<li>Ç - Ç</li>
<li>Ğ - Ğ</li>
<li>İ - İ</li>
<li>Ö - Ö</li>
<li>Ü - Ü</li>
<li>Ş - Ş</li>
</ul>
</body>
</html>
This code defines a basic HTML document that contains a heading, a paragraph, and an unordered list. The list items use the HTML character codes to display the Turkish letters. You can copy and paste this code into a text editor and save it as an HTML file, then open it in a web browser to see the result.