I've compiled a list of websites that I know sell "Invitation to a Feast". They are linked to the appropriate purchase pages. Here they are:
Ark House Press (my publisher) Australia & New Zealand
Ark House Press (my publisher) U.S. & International
P.S. Koorong also have a section where you can write a review on the story. If you have already read it and enjoyed it, I would love it if you could write a review (even a really short one) at the Koorong link!