With Arianna's Sesame Street party in mind, I decided create a fun polka dot pattern that reminded me of the scrapbook paper I used on one of the decorated tables. I loved this raised, polka dot paper that I purchased at Joanns and was so happy that I thought to include it at the last minute.
I found a great tutorial about creating simple patterns in Illustrator that can be found at Design O'Blog and made a very simple polka dot pattern that I made into an envelope seal on.
Polka Dot Envelope Seal Round Stickers by AnderBerry_Creations
Order cheap custom stickers online at zazzle.com
But if you don't feel like and/or have the time for creating a design from scratch I highly recommend checking out Cupcake Cuties and Pixel Scrapper. They've got great graphics with some available for use on print on demand sites like Zazzle (be sure to check out their Terms of Use).