The 16 free DLC will be available to all who buys the The Witcher 3 game.
The Season Pass will at some point be available for all to buy digitally. You can already buy the Season Pass from GOG.com and Steam. You can also buy the Season Pass from Xbox Live, PSN, Origin, although I am not sure if the Season Pass has been made available on those market places yet but it will be there soon.
The Season Pass (expansion packs), at this stage, will not be sold in retail. I am hoping that CDPR will at some point sell the expansions in retail. But no word on that yet.
Retailers like EBgames and Gamestop will say anything just to get you to pre-order (reserve) the game from them. That is why retailers love exclusive game content. To get people to buy from them.
No game content for The Witcher 3 is exclusive to any retailer. There are pre-order incentives like key chains, steelbooks, posters, comics, ect, that will be exclusive to some retailers.
This comment will be deleted because I am too sure of myself. LOL. Thank you mods. So you will have to ask CDPR to answer you about EBgames selling the Season Pass, exclusively at retail. I am not CDPR therefor I can not answer you. Thank you mods.