Outside of Tamer bags, I think your best bet would be to battle some wild Undead Pets. The higher level wild pets have a better chance for dropping a stone, but the highest level Undead are the Unborn Val'kyr
and Scourged Whelpling
. Both are likely scarce so the next highest is the Restless Shadeling
The more abundant Undead pets are Infected Fawn
and Infected Squirrel
. Not as high level, but I wouldn't count them out.
Hope you find a stone for your little valk soon.