Most of the commercial worm castings are below average products (Wiggle Worm especially). The bigger producers feed the worms a steady diet of shredded paper and card board. Worm castings are only as good as the feedstock they were provided. Your best action is to start your own worm bin. This is as easy as buying some 20 gallon Rubbermaid containers and some red wigglers. A steady diet of fine organic waste products and great compost will give the most nutrient rich castings you have ever worked with. There is an extensive thread about this subject over at Grass City
http://forum.grasscity.com/organic-growing/334921-vermicomposting-make-your-own-worm-castings.html. This thread will blow your mind and make you rethink the way you garden organically. If you don't have the patience to read a long thread and think good enough is fine there are two brands that are recommended among some of the elite organic farmers, Vital Earth and Coast Of Maine. Also there is a company in Michigan called Morgan Composting that looks like they have a solid product as well
http://www.dairydoo.com/Morgan_Composting/Home.html. I cant say enough about the quality of a fine home made worm bin casting.