[sufue]

I did a very similar thing as AT Drake suggests.

I used the Stop You're Killing Me website, and just checked through the website's bibliography for Agatha Christie book by book, looking first to see if I already had each one - clearly I don't have perfect control of my collection of ebooks! I then just searched the Kobo site for the few I still needed. That worked pretty well since I already had a lot, and saved having to page through the huge numbers of individual short stories that pop up, as others have noted, if you just search on Agatha Christie and the sort by price.

One note of caution - a few of the books I needed were short story ***collections***, and in many cases the title of the book is "The Lead Story and Other Stories" and there is also then an individual version of just "The Lead Story" available. So in that case, you need to be careful to pick up the book version which is the collection, rather than just the individual story, since right now they are both 99 cents in Canada.