USDA Feed Ingredient Analysis Search https://ndb.nal.usda.gov/ndb/
Two pretty exhaustive documents on nutrition for dogs and cats. http://dels.nas.edu/global/banr/petdoor
Cats: http://dels.nas.edu/resources/static-assets/banr/miscellaneous/cat_nutrition_final.pdf
Dogs: http://dels.nas.edu/resources/static-assets/banr/miscellaneous/dog_nutrition_final_fix.pdf
Excellent jumping off point to start looking at home made diets, cooking for dogs: https://thebark.com/content/10-myths-and-misperceptions-about-homemade-dog-food
Nutritiondata Interesting place to help interpret ingrtedients and caloric requirements etc. https://nutritiondata.self.com/