There are federal regulations regarding what can be labeled organic and not...and there is a marking they put on products. You'll notice it.
I tried the Target brand organic milk when I picked up groceries last week, and thought it was good enough...though it did end up going bad before the expiration date. Odd...
I think that the Horizon organic milk is good, too, but find that if I can get locally produced milk, it's much more flavorful. Might see if anywhere around you carries organic or at least natural milk (like w/o hormones but maybe the cattle eat feed that's not necessarily organic).
Whole Foods, Wild Oats, etc. have their own store brands of nearly everything, organic milk included, and I find its qualities to be similar to the Target brand, similar to Horizon's. It's certainly cheaper than name brands, you'll have to decide whether the taste is any different to you and whether it's worth it. If you're going to Whole Foods or WO, see if they don't carry locally produced milk, it's my best recommendation.