What meaning of FOR in "stumped for an answer", "lost for words", etc. I've gone through the OED looking for the right meaning to no avail, as well as other major dictionaries. https://www.oed.com/oed2/00087678
This dictionary definition may fit: Merriam-Webster "for"(7)
7: with respect to : concerning
a stickler for detail
heavy for its size
Another use from the same definition
3 because of
would work as well, if you interpolate "lack of" in the phrases.
So your examples can be paraphrased as
Stumped concerning an answer or
Stumped because of [lack of] an answer
lost as to words or
lost because of [lack of] words.
Note that not all possible uses are enumerable in a dictionary.
Also, check out 23.c. in your OED reference:
c. Indicating the presence or operation of an obstacle or hindrance.
(again with "lack of")