I buy lab block in bulk because of the quantity of mice I raise. I would recommend Harlan Teklad's 8604 blend. I think if you buy it from them you only have the option of buying a 33 lb bag, but there is a website that sells them in smaller bags.
I recommend getting a good lab block to use as your staple food. Try to avoid blocks that have corn as the main ingredient. Corn used in most rodent food is not food-grade, but instead commercial-grade. Brands like Kaytee Forti-Diet have MUCH too high protein (about 21%, when a mouse should not get more than 13% daily) plus the main ingredient is corn; however, because it is readily available at just about any pet store it's very popular. If you want the most out of your pet, take the extra step and order a good diet online if you can't find one in the stores. Don't settle.
I give a seed mix, which I make myself <with seeds, oatmeal, plain Cheerios and dried unsweetened fruits), three times a week in moderate amounts. I also give them veggies daily, not so much fruit though (mainly apples and grapes). I buy frozen peas and corn nibblets, thaw a little bit out in a cup of drinking water, and place in a paper towel to dry before giving them to the mice at night. I also give them baby carrots or shredded dark greens in small portions.
P.S. they ABSOLUTELY looooove the snow peas! One small bag of frozen peas will last FOREVER if you only have one or two mice. I'd give them 2-3 peas each.
For treats, I use the occasional spray of millet (like you feed birds) or training size Milkbone dog treats.
Mice shouldn't get peanuts and other nuts very often, only as an occasional treat. Try to avoid onions and citrus and chocolate.
Hope that helps!