The dollar store is a good idea, I've seen them there.
Check with your librarian. They usually know of great resources.
Also speak to the managers of your local book stores. Maybe they could help out.
Definitely key an eye on Bookcloseouts.com
I checked their current inventory and they have 100 copies in stock of
The New International Webster's Dictionary
for 49 cents each. And always Google them for promotional codes. They are always running something, like $10 off a $50 order. If you go into the kids' area and "type dictionary", you'll find some more choices.
Student Notebook Dictionary (Second Edition)
is $1.99 and they have 100 copies.
Several of our teachers have asked for individual student dictionaries for their classrooms. Can you suggest a vendor who can supply good dictionaries at a reasonable cost for 160 kids (grades 3-6)?