you can only get them on the street corner in big citys from a guy named "pointy". When you go to buy them you have to call them "8-balls" so the postal police don't catch on.
You could get them off the internet. I think there's even a site where you can pay and print them up yourself.
1.United States Postal Service [New Window] Find zip codes, track delivery, or calculate postage. http://www.usps.com 2.Stamps.com [New Window] The Stamps.com Web site. http://www.stamps.com/ 3.Postal Center [New Window] Buy stamps online or track a package sent by postal mail, UPS, or FedEx here. http://portal.compuserve.com/wmconnect/comm/nocl/postal.asp