I'm not actually too sure as I'm away from home at the moment but I just buy it from my local whole food store. (deliciouslyella.com)
I don't actually use a specific brand but I'm sure anything in your local whole food store would work fine xx (deliciouslyella.com)
I've seen wheatgrass at my local whole foods store but its not cheap and if you're using a centrifugal juicer the majority of wheatgrass is wasted. (justonjuice.com)