For me my Carbs come from; Oats, Sweet Potato, Fruits (Banana's are great for easy carbs), Rice, Pasta and Vegetables. Weetabix are also good, I often take some into work and have them with Milk or Whey. - and it means less cooking. Honey is also a great carb source if you're really struggling to eat enough.
It you're OK with nuts, Nut butters are a great way to get some Fat. Delicious on some toast or a bagel(another great carb source!) Peanut Butter is a personal favorite and can be bought fairly cheaply, although the less added stuff the better. Meridian do a 1KG tub for a fiver depending where you buy and they last me a good while. Meat wise, tinned Salmon is a fairly cheap healthy fat source. Eggs are another great option and are also cheap; you can boil some the night before and then just throw them in a Tupperware for during the day. Dairy is also a good all-round source of Fats, carbs and Protein.