If you want to build your stash of coupons - tell the company you want some! Whats the worst they will tell you (well, besides NO)? I will start posting companies as well as the link for you to ask for coupons.
My letters typically go:
I really would like to try your XXX product. I was wondering if you would be willing to send me free samples or coupons for my family to try XXX product." Signed with my name and address.
Here is a few to get started with: