Silhouette Studio allows you to buy or create your own designs...but their choices are also somewhat limited.
Here is a way to easily create your own elements using Paint.NET (Free to download, can substitute photoshop as well) and Silhouette studio (also should work with other die cutters or stop after step two, print, and cut out by hand).
Picture of design you want
Silhouette and software
Step 1: Find a Design You Like
Since I don't have a picture myself, I'll be taking a screenshot, which you can do by using ctrl+printscreen and then pasting into Paint.NET or another program.
I'm going to crop it so that there's less in my image, which will make step two easier. For this step, rectangular select and crop to selection tools work fine.