Can this be accomplished by aging saltwater out in the sun ? Are there any methods besides buying a greenwater culture kit from someone like florida aqua farms?
Have you checked the Advanced Aquarist Index? I know that Frank Marini recently had an article in there about growing greenwater at home. Check it here: http://advancedaquarist.com/index/ and click on the The Breeder's Net (Frank Marini) (Martin Moe) link.
Joyce Wilkerson discusses outdoor cultures in her book Clownfishes. I can't recall if she discusses outdoor phyto cultures, but she does mention outdoor rotifer culturing.
Not sure that really helps, but at least you get a bump... :?
Yeah, that section in wilkerson's book is for rotifers, not phyto. I wish setting a bucket outside in the sun was all there was to it! Not that culturing it is necessarily difficult, I just don't have a lot of space.