Here is something I posted some rime ago.

"If possible buy from professional level companies that sell through physicians and health professionals rather than health food stores. Some brands include Allergy Research Group, Douglas Labs, Designs for Health, Pure Encapsulations, Metagenics, Physiologics, Ayush herbs, EuroMedica, Lane labs, Quincy Bioscience, Gaia herbs, Innate Response, Protocol for Life Balance, Perque, Klaire labs, NOW, DaVinci labs, EcoNugenics, Enzo, and many others. Life Extension Foundation is also excellent. "

Quality manufacturers typically advertise on the labels, if they are GMP or USP... NOW, Jarrow, and NatureMade, and Doctor's Best are examples of quality products. I am sure there are others, but these are what I tend to buy for our home. I also use Puritan's for some of the common supplements, like, C and E and some amino acids.