Disney has a presence on Zazzle, and a multitude of Christmas ornaments. In fact, if you are searching for any character there will likely be an entire selection for that character. And, Frozen ornaments can certainly be found.
First, select the image. Once the ornament is clicked on there will be the option to customize it. This feature allows you to add a name, select the placement of the added name, and even select the size, style of lettering, and color that name will appear in. But, before adding anything, select the style of the ornament itself if allowed. You can select the round ornament, the one that first appears, a star, a heart, a shirt, and an oval. Not all shapes are available for some ornaments.
The reason the shape of the ornament is first selected is to prevent a problem with whatever you add. You do not want letters dropped from the name, do you? A name written across the top of a heart might just have missing letters.