I have found the best location to find Star Wars mission expansion books.  These are the books that come with scenario and extra maps for your Star Wars miniatures game.  The best place to look is Amazon.  I know most will think that Ebay is the best place to find them.  Remember though, that sellers on ebay are trying to make a profit.  These on Ebay are retailers that are trying to cut losses.  I have purchased both the Rebel Storm and The Clone Strike expansion for $3.00 each.  I have looked around prior to making these purchases and found them retailing at $29.99.  They are found on Ebay from $19.00 to $40.00 each.  So check Amazon to find these they are becoming as rare as the figures.