I cannot find any store that sells them. Please help.

As far as I know there is no retail version for the addon. Perhaps in future.

There is, just watch the latest trailer.

Ah i see. But it is only a Limited Edition with a Download-Key and some Gwint cards.

"Although primarily a digital release there will be a limited boxed edition available exclusively at GAME (although it doesn’t seem to be listed on their website at the moment)."

Read more: http://metro.co.uk/2015/09/08/the-witcher-3-expansion-hearts-of-stone-out-next-month-plus-real-gwent-cards-5382047/#ixzz3lGSoUdvC