There's a bunch of suggestions in the threads around here, but the key word to search for is generally "salmiak" [1]. You can order just about anything on Amazon these days, but if you wanted to check for local stores, try for specialty candy shops, especially those with a traditional nordic background or an adjacency to a major shipping wharf.
(I learned about it myself during a photo scavenger hunt in Seattle in college. I'm still amused by the photo from that where the rest of my scavenger hunt team was dying from taste, and I'm the only one enjoying it. There was a well known wharf candy shop we were directed to as the easiest place to buy it and get that photo op [and they were used to and amused by college kids making the stop].)
