Kellogg's website for some great high value coupons. If you have not already done so you will have to sign up for an All-K Pass which lets you have access to any of the Kellogg's family websites.
The current coupons on Kellogg's is:
$1/2 Poptarts 8 count or larger
$1/2 Raisin Bran or Mini Wheats 14 ounce or larger
$1.50/3 Any Kellogg's cereal 10 ounce or larger
$1/2 Any Kellogg's cereal 10 ounce or larger
$1/2 Kellogg's Special K protein products 4 count or larger
$1/2 Special K cereal 11.4 ounce or larger
$1/2 Frosted Mini Wheats 14.5 ounce or larger
One of the places that I get some of my high value coupons from is Manufactures' websites. They are a great source of ongoing promos and coupons that you sometimes can't find anywhere else. Also, sometimes when you print your limit of a coupon from Smartsource, Redplum, or Coupons.com you can print an additional 2 from a manufacturer's site.