Seed mixes don't provide proper nutrition and the pesticides used in corn tend to cause cancer. Avoid products with corn high on the list. First ingredients should be more along the lines of healthier things. Soybean meal would be a great first ingredient, as it helps to prevent mammary tumors.
I order Harlan Teklad lab blocks online. Regal Rat is the best food, I order HT because my girls are quite young and have different needs. HT offers a kind especially for growing rats.
Oxbow Regal Rat is good, and is sold at many chain pet stores if your rats are adults.
If you absolutely can't get your hands on some lab blocks without a lot of corn, I would recommend making a home mix. Here is a great recipe:
"Suebee's Grain Mix
As a reminder, Suebee's mix is half staple diet and half grain mix. Don't worry if you can't find all of this in the exact sizes, this mix doesn't have to be exact. Also, you may have to substitute some products with others, just try your best and you should get a decent mix. I'm usually able to get all of my stuff at a local supermarket, but if you have a health food store it's probably better to check it out first (Though it will be more expensive). Also keep in mind that this makes A LOT.
1/2 to 1 lb rolled oats - With the oatmeal at the store
5.3 oz boxed puff wheat cereal - Sugar free! Quaker, Malt-o-Meal, or Kashi. Cheerios could substitute if needed.
6.4 oz. box puffed rice cereal - Same as above. Quaker is recommended
12 oz. box Total Cereal
1/4 to 1/2 lb. roasted, unsalted soy nuts
1/2 lb dried fruits - Be careful here with fried banana chips and the like. You can get freeze dried ones online, or in the baby food aisle they sometimes have freeze dried fruit, however this is expensive. Fresh fruits could work too
1/2 lb dry pasta - Elbows or whole-wheat pasta
1/4 lb sunflower seeds - The black and white unsalted ones that are for birds. I have the hardest time finding these, but good luck!
Optional - 1/4 lb muesli
Just mix everything together in a large bowl or multiple large bowls and store in an airtight container. It should last for a long time!"