I was looking for a diamond shape background stamp recently and couldn't find anything close to what I wanted. Granted I don't have a lot of rubber stamp stores close to me so this stamp might exist somewhere out there. So I decided to make my own using craft foam cut into diamond shapes, a hard backing sheet and used tacky glue to glue the diamonds to the backing. It's worked really well and I've used it a few times.
After having success with the diamond stamp, I made two others. One is a circle stamp and the other is stars. Again these are meant to be background stamps and therefore look rather rudimentary. Here are the stamps I designed.
Here are some of the backgrounds I made with the stamps: