In the scrapbooking aisle! Get a paper crimper, break it open and use the 'roller'. As a bonus - u get a raised 'roller' as well - (haven't tried that yet, but might be fun!).
There are so many different designs...
To practice decorating, invest in a few cake dummies. Practice that new technique on the dummy, dont forget to take a picture of your work, and then you can wash, or peel away and reuse. Saves on the ingredients...