I got mine online at Amazon. I know you stated besides online but it's worth mentioning that if you get the iPhone 4 version, it costs almost one-third less than buying the PMA charging pad itself. I just threw away the iPhone 4 cases.
Update: That said, I just looked at Amazon.com (I had to order from Amazon.ca) and the prices seem more in line. $14 for a PMA 2-device charging pad, not bad at all, better than what I paid for mine through .ca. They wanted $80+ for some of them.