www.mysimon.com (compare prices on items)
www.froogle.google.com (I don't think this one works well)
Also, I've used the "needle in the haystack method." I've been ready to check out from a website, and realized I "could" use a code. When all my resources turn up no codes, I'll google "store name" "coupon code" or "discount code". Often you need to really scour to find one, but, usually there is one out there!