Is there a place to find decorative stationery in Pages ?

Apple Mail has stationery, but it is web based content, and unsuitable for Pages. Pages has no decorative templates. Here are your options:

1. Find a PDF of the type of decorative fill that appeals to you, and drag/drop it onto Pages. Then, from the Arrange menu, you make that PDF content the background by Moving it to Section Master. Take a look at openclipart.org and search for words to describe what you seek. You can download as PDF. Keywords: decorative, stationery.
2. You locate a MS Word stationery template (that does not use Word macros), and open it in Pages. Then save it as your own custom template, for reuse with your poem.
3. There are purchasable Pages template packages available in the Mac App Store. Visit the vendor's web site to review to your needs. Verify that they are for the version of Pages that you are using.
4. Make your own Pages stationery design in a Pages document, and save/reuse for your poetry.
   1. Visit Subtle Patterns and choose your pattern for the background of your page. Then View menu : Show Layout, and toss in a rectangular shape, or text box. Stretch it to cover the entire page, and then from the Style tab (Pages v5/v6) select Fill : Image Fill : Scale to Fill : Choose ... and select your subtle pattern. This will work for smaller shapes as well.
   2. Arrange menu : Section Masters : Move Object to Section Master to make it the background for your text that will float above it.

Do we know where you saw it and what you saw?

Well no, have you looked in the App Store?

Peter