At one time, Microsoft had a pretty nice collection of custom Outlook forms anyone could download. While a number of the forms are no longer available from Microsoft, a sizable collection remains. You can use these as-is or as a basis for your own custom forms. See //www.slipstick.com/addins/mssampleapps.htm for the forms that are available.
While they were all designed for Outlook in the Outlook 97/98 time frame, every one I've tried works with all versions of Outlook. However, the self-installers may not work due to security settings and you'll need to publish the form yourself. If you need step-by-step help to publish a form, see https://www.outlook-tips.net/howto/publish_form.htm.
Published July 12, 2004. Last updated on May 8, 2014.