Donations are best from friends and family. Jump on Facebook and see if you can get some feedback. The last census was only 3% of circulating coupons were being used. I bet you can find a lot of that 97% willing to help. Ask teachers at school. Many look for newspapers for projects. I'm wondering how many happen on spare coupons as well.
I personally purchase large quantities on eBay. You can get several (from 5 to 20) of one coupon or even buy multiples of inserts.
As for newspapers, they are required to destroy any left over coupons. At our local Walgreens, I overheard the manager repeating the same thing for unsold papers carried in their store. I would forego these outlets and stick with family, friends, and eBay. (And recycling bins if you're brave...) Good Luck!