No, there's nowhere that someone who isn't a security professional can download such a thing. MacDefender is extinct at this point anyway, so you won't find it in the wild anymore, either.
If you are curious about the history of MacDefender, you can see the coverage on my blog from 2011:
(Note that I have provided a URL that sorts the posts from oldest to newest, which makes the "Older entries" link actually display newer entries. Hmm, guess I need to fix that. )