Probably the dealer. If you're not close to one, they'll ship it to you.
There aren't a ton of these out there being parted yet. A professional yard isn't going to fiddle around with something like this. If you want to buy the door, they'll include it. Finding one of these in a pick-a-part yard, well... just buy a lottery ticket. Probably the same with Ebay.
You better hurry- virtually everyone is purging all their parts inventories after ten years, except for the regular sellers that are profitable to keep in stock. This wouldn't be one.
Good luck and welcome to the board. Do yourself a huge favor and start researching maintenance requirements and issues on here.