In the olden days, documents used to be signed and sealed with wax. While that particular practice is no longer en vogue, the corporate seal is still very much in use today.
Nowadays, the meaning of the corporate seal has shifted somewhat. Whereas in the past, it was used to differentiate the official imprint of a company from the signature of a director, it currently has a more symbolic meaning. Even if no longer legally required by the state in which you have incorporated (consult with a lawyer to determine if this is the case), a corporate seal can confer an air of authenticity to any communication your company might send. Here's how you can get one:
First, your corporation has to be registered with the state. This is key: because corporations are state-based entities, your seal will likely bear the name of the state of incorporation.
Next, you have to design your seal. Because of the decreased legal significance of the corporate seal, you have some leeway here, but most tend to bear the same information. Useful things to include are your company's name and the date and state of incorporation.
Finally, you have to choose an embosser, and have the seal built. The actual stamp is built into a stapler-like device that will allow you to apply it to whatever you see fit. Decide how frequently and in what capacity you will use the embosser, and then choose an option that fits in with that plan. Factors to consider here include price, style, portability and build quality.
If you're interested in getting a corporate seal and would like more information on the process, an incorporation services company can help walk you through your particular requirements.