Place to find a large quantity of cheap textbooks?

I am looking to start a small business that will help raise funds for animal shelters in my area (my New Year's Resolution). I have sold my textbooks, and now I am looking for a source of additional textbooks so I can continue this and continue to donate money to my local animal shelter (which they desperately need!). Any hints? Websites? Advertising that works?Place to find a large quantity of cheap textbooks?
The most effective way is to engage the help of your friends to perform house to house collection.

Most if not all, won't take the trouble to send to you.Place to find a large quantity of cheap textbooks?
I would put up flyer's on the bulletin boards of colleges, Jr. Colleges, and local hangouts of students. If there is a "free" local paper or magazine, you could put an ad in there too. Lot's of people would like to get rid of those old books gathering dust on their shelves. You might try flea markets, too. Good luck.