How do I find who is responsible for making purchaces at colleges and big company's?

I was trying to sell my christian apparrel to bookstores at christian colleges but the managers never seemed able to accomodate me. someone told me to skip over them and look for those responsible for the purchasing.....how can I do this.How do I find who is responsible for making purchaces at colleges and big company's?
I'd start with the bookstore manager, and if they can't help you ask them for the name of the person who can.



But they might just not be interested, and are trying to say "no" politely.