you should check out OWASP as it directly relates to your presentation. Specifically to answer your question, you should check out OWASP WebGoat project is its is a deliberately insecure web application designed to teach web app security. (And contains XSS vulns, among many others) You can download it from here: https://www.owasp.org/index.php/Category:OWASP_WebGoat_Project
Additionally, it isn't difficult demonstrate an XSS vulnerability if you have basic programming knowledge. (PHP,ASP,Python). Create a page that asks for input, have it post to a second page that takes that input and displays it back on the page. In php it would be something like this:
index.php:
<?
if ( isset($_POST["posted"])) {
$name = $_POST["name"];
}
?>
<html>
<body>
<? if (isset($name)) {
echo "Hello, <b>" . $name . "</b>";
?>
<form action="index.php" method="post">
<input type="text" name="name" value="">
<input type="hidden" name="posted" value="1">
<input type="submit" name="Submit" value="Submit">
</form>
</body>
</html>