Gogamer.com used to carry the retail PC version of DKS, but they are temporarily closed. If some of the US Gamestops carry the game (new), though, I'd think any of them should at least be able to order it in.
You can activate a digital version of the game offline. When the copy protection fails to detect an internet connection it will give you the option to request an unlock request code, which you can copy and email to Larian (from another computer) along with your activation code / serial number in order to get an unlock code sent back.
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