All the Pandora from the first batch are already sold out.
Openpandora will be taking pre-order for the next batch at the end of this month. http://twitter.com/Craigix/statuses/15513907116
I recommend you subscribe to the mailing list on http://www.openpandora.org/
Then there's always ebay but that just crazy right now if you ask me.
Also check the where to buy on the official webpage:
http://www.openpandora.org/index.php?option=com_content&view=article&id=97&Itemid=34&lang=en
There is no US reseller.