There are lots of sunflower varieties, if you choose a smaller variety I don't see why you can't use growbags, you might want to turn the growbag the other way around (on it's end like a sack) growing one or two plants, choose a branching variety for lots of flowers. I grew a couple of Autumn Beauty in a large pot last year and they grew beautifully and didn't seem at all bothered being in a pot. :)