Start by going to Feathersite.com or getting an APA Standard of Perfection book and learn about the specific breeds that you are interested in. Learn what they should look like and about their specific features for comparison. Then look for a breed club, through them you should be able to find reputable breeders.
Many large hatcheries may very well have pure bred birds, but that doesnt mean that the birds meet the standards for the breed.
Also, attend poultry shows and go look at the actual breeds that you are interested in. While at the show, go to the sales room and see if anyone is selling some of the breeds you want. Dont hesitate to ask a bird owner or seller to point out both the good qualities and the less desirable characteristics of their birds.
Technically, with chickens, if the bird meets the standards for a specific breed then it is said to be that breed, whether it is pure bred or not.
chel