Sunday newspaper- Generally, the Sunday newspaper (and only the Sunday edition) has 2-3 coupon inserts. These include the Red Plum and Smart Source inserts. Occasionally, Procter and Gamble includes an insert as well (I believe it's once a month). Weekends of holidays, the Sunday paper does not have any coupons. The cheapest way I've found to buy a paper is by subscribing to your local Sunday only paper.
Internet Printables (known in the blog world as IP)-
Coupons.com is great source for coupons. They upload new coupons every month and allow you to print two of each coupon.
BettyCrocker.com- You do need to set up a free account with them.
Some brands have their own coupons as well. If I am looking for a coupon for a specific brand, I type the product into my search engine followed by "coupon" (ie: Chex Mix coupon) and see if it comes up with one.
Trading Coupons- I haven't done this myself yet, but I know there are places where you can trade or buy coupons from other people online.
Catalinas- A Catalina is a coupon that prints out at the register. It will often be for a specific product or a certain amount off your next purchase.
Free Samples- Free samples usually come with a coupon for that actual item.
Blinkie Machines- Those little machines that spit out coupons in front of products at the store...I find that when those are out, that item tends to go on sale soon!
Tearpads- Like the blinkie machine, these are pads of coupons found in front of an item at the store.
Coupon flyers- Found at the grocery store.
Store coupons- Either found on the store's website or in a flyer at the store.
Requesting a coupon- Sometimes a company will send you coupons if you request it from them.