Take last week for example....I ended up with 20 boxes of Ronzoni pasta for free....how in the heck did I get 20 of the same coupon? Well that particular situation, I found one $.75/coupon in my sunday paper inserts. I knew that the pasta would be free at my grocery store using that coupon so I went to ebay and bought 20 for $.99 because I was almost out of pasta. So ebay is one source for multiple coupons. You can usually get them in quantities of 10, 20, 50 for a small percentage of the face value...usually around $1 for 10. If you're getting a product for free after the coupon, that $1 investment is no biggie.
So besides your sunday inserts, and ebay....you can find coupons in the stores themselves. I'm sure you've seen those blinkie machines hanging on the shelves....or tearpads on a display for a new product. You've probably even seen stickers (we call them peelies in the coupon world) on the actual products themselves. You'll find all of the above at grocery stores, drug stores, gas stations, convenient marts.....really anywhere. The coupons are put out by the manufacturer, not the store themselves.
The newest version of coupons are those found online. You can usually print coupons directly from a manufacturer's website. In addition, sites such as coupons.com or redplum.com offer a large variety of online coupons. You can find a large list of printables in the database at www.afullcup.com
Trading with friends and/or using coupon boxes are local libraries, day care centers, churches are other options.
Really, you're surrounded by coupons, you just have to start noticing them. :)
Happy shopping my friends :)