The FIRST way to purchase coupons is through your Sunday paper! Buy the Sunday paper, and BUY COUPONS!
The Sunday paper is the most common place to purchase coupons! Coupons are in inserts which are in most Sunday papers with the exception of 4-6 weeks out of the year.
The SECOND way to purchase grocery coupons is through a distributor. Some people purchase large amounts of coupon inserts and are willing to part with coupons they are not using. You can find those here: https://www.coupons4sale.com