DKS did not get a retail release in the US or UK, but you should be able to order it online if you want the disk version. Steam and Amazon (etc) occasionally put the download version on sale.
GoGamer.com in the US used to carry DKS, but the site is down (there used be a messages saying they would be back 'soon', but now it is just a logo). AFAIK nobody in Canada that carries the game (Amazon/Gamestop) ships outside of Canada (and Gamestop no longer lists the PC version on their site).
A few people in the UK mentioned ordering from Play-asia.com (though it is out of stock there now, as it was last time I checked, about 5 months ago). You should be able to order from any European country (except Germany), since they include the English language version of the game (but not necessarily an English manual).
I can't buy a hard copy?FoV is available as digital download only (just the German version got a retail release). Steam elected to only carry DKS (with a discount for anyone who bought D2:ED through them), and other digital distributors who did have FoV may no longer list it. FoV is still available directly
from Larian, though you may be able to get DKS cheaper (Amazon.ca currently lists the retail version of DKS at CAN$22; with sales, download versions have been as cheap as US$8-10).