Commercial rodent food... all you need for rats. They don't need any supplement as long as you get a good quality rodent diet. Mazuri, ADM, Harlan, even Kaytee isn't too bad... The only one I'd stay away from is Kent..
All of these may "sound" expensive, especially when you only have a handful of rats, but in the long run, it will save you money.
You should be able to go to a farmer's co-op and order from them if they don't carry it. Shouldn't be more than $30 for a 50# bag. Most people can find it for around $20-25.
Or, do what some of my friends do... They find a breeder(like me), who's nice enough to order a few extra bags when they order... Comes out even cheaper, like $17-20 a bag..
EDIT: By Kaytee, I mean, Kaytee rodent blocks, not the crap seed mixes...