Fortunately the parakeet seed bags I have been getting already have a variety of seeds in them, although I am sure the variety could be better. I have not seen additional seeds sold individually available in local shops. Maybe my local pet shops are just too small for that. I usually just pick up a bag of parakeet seeds at the grange and add millet now and then.
My birds are kind of picky eaters though. I tried so hard at first to get them to eat veggies like the books recommended, but they simply turned their noses up at it and never even tasted it. I don't know how I can convince them to eat it if they don't want to.