If you are looking for a virtual seal or you want to give an idea on how seal has to be generated to your designer, Official Seal generator is a neat tool which you can use. Based on most common pattern and dice used for generating the seal i.e. lets you design a custom seal with your text, colour, embed options, custom image etc.
Here is a sample seal for pentagon shape and you can see the detailed customization. At the end you can have the seal downloaded for printing or forwarding to your designer. Check out Official Seal Generator