Problem with most of those materials and atmospheres, it's same material, just a modifications in coloring.
From all of them i recomend (i have it) :
Cloudscape pdf on usage plus 12 cloud layers and atmospheres for $14.90
http://www.cornucopia3d.com/purchase.php?item_id=8216

Mats and atmospheres may look good in demo render, but not realy usefull for any thing else.

Geekatplay mats, i created for users with version Vue that does not have function editor, if your version does, i recoment watch tutorials at geekatply.com and create by your self.

Also many free mats, pressets, atmospheres..etc you can find here, at renderosity.com, geekatplay.com and cornucopia3d.com Gill Brooks creatins are very nice.

If you find product you like, look for some examples that user created with it.