The unlocked version of Samsung Galaxy S3 just went on sale 2 days ago, but was now currently out of stock according to Samsung’s website. Main reason for the shortage of stocks is probably because Samsung and Verizon both did not foresee such a huge demand for this device. According to Samsung’s website, interested buyers could actually place an order on their website, as the stocks are expected to arrive only in the next 2 weeks.
Just for your information, there is no differences between the original Samsung Galaxy S3 and the Samsung Galaxy S3 developer edition except for an unlocked bootloader meant to increase the ease for developers to code their applications.