You can buy one from any place that makes rubber stamps but of course you need to give them the correct design that follows the rules set out in the Postal Regulations and so would be accepted by the Post Office. There are a couple of guys in the Mailer's Postmark Permit Club who make and sell them as well. Go to the MPPC webpage and in the FAQs section, FAQ number 9 includes a couple of email addresses for two of their members who make and sell them as well.
http://www.mppclub.org/FAQ.htm You can look through their various pages on their website to get the full information on Mailer's Postmark Permits and how to use them. You can also join their membership if you decide to get one but it is not a requirement.