A single copy of Walk in Shadows turns up much
But as a general rule, it seems that if you want Walk in Shadows, you'd better select it at game start.
I checked all vendors in the location named in the first spoiler, and it is seemed as if each has three unique spells, or at least spells that I haven't seen available from any vendor yet despite frequently reloading to figure out how the vendor spell restocking system worked:
I have not finished the game and am only at level 11, so I cannot say if the spells really are unique and only available there, but I certainly hadn't seen any of the listed spells at vendors before despite many of them being of lower skill level that the highest skill books I was at that time being offered at the regular vendors in Cysael and Silvergleam. (With 4-6 reloads to check inventory variance at each seller each time they restocked).
Anybody who manages to get get Whirlwind on two different characters (or Phoenix Dive, for that matter), please tell me how as I'm dying to know whether the second man-at-arms in my four-person group will have to go the rest of the game without ever realizing his full potential. If you bought them from a vendor, what was your exact level when they became available?