Check this out... on Deere's website, when you try and build your own, you get the choice of the machine with a bunch of accessories and options to add, but you cannot order an attachment (mower deck not included, contact dealer for attachments). That means the dealer has to order the machine with whatever deck you want. It would seem the dealer could order just a deck if they can order just a machine, wouldn't it?
I still suggest you try another dealer.
/ John Deere apparently does not sell Mower decks for some of it's current model machines.