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.
Step 2: Remove Everything but the Design
The easiest tool is the Magic Wand. The magic wand tool selects all of a color (we'll use black) within a certain tolerance range.
I've turned my magic wand to global, so that it will effect the whole picture and I've turned up the tolerance to 62%. Now, I'm going to click on the black picture frame since it is the same shad as the art. It should select all of the heart. If it does not, double check your settings and increase tolerance.
Next, we're going to invert our selection (second picture). This will mean everything that is not black is selected. Once done, delete the selection (third picture).
Since there still is a little unwanted material in our picture, I'm going to crop it using the lasso select tool (fourth picture). Draw around your shape til everything is included and then crop to your selection (fifth picture).
Save the file as a PNG.