Arnott's Mint Slice Cookie
Perhaps the places that carry Tim Tams also stock Mint Slice?
The ones I'm aware of (though you should call to ensure its in stock) are:
Tuck's - there are 2 locations but the EV spot on 1st Street off of 1st Ave seems to stock a larger selection of Arnott's.
Holyland Market - 122 St. Marks Place 212-477-4440 They are usually the most consistent source
Landmark Sunshine Movie Theatre on Houston at the snack counter
and of course there's always the mail order option such as http://tinyurl.com/3lb23y
good luck and hth