Cheerios are good, make sure they're just plain. Rice crispies are good too but make sure you're just getting puffed rice and no added sugar as well. Raisin bran might not be good, the flakes are probably healthy, but the raisins are probably junk food to mice. Also most raw veg should be given very sparingly, because mice are not really made to digest vegetables.

Here's an approximate idea of what a homemade mouse mix should be...

60-70% grains (oats, barley, whole grain pasta, etc. Avoid corn)
10-20% holistic dog food (nature's recipe I think is a cheap holistic food)
10-15% seeds (millet, pumpkin, thistle etc. Avoid sunflower seeds)
10-15% lentils (any colour lentils are fine)


The numbers are aproximate, but it gives you a good idea of portions.
You can go to a feed store near you, and ask them to order it into the store. You don't need to pre-pay, just go pick it up when it comes in, and you can pay cash. You can also do what I do, and buy 'sow and piglet' hog food. It has almost the exact same nutrition as rat block, just in smaller pellets. Make sure you pick out the hog food with the highest protein content (I think mine is just right at 16-18%). It's about $15 for 50lbs, so it's comparable to lab block in every way. I pick it up at tractor supply once a month or so.