The American Speech, Language, Hearing Association (ASHA) provides information on each of the different communication disorders. This link will take you direct to their page regarding articulation and phonological disorders. It provides information about signs, causes, testing, and treatment.
This webiste is also a great source for free resources including: flashcards that can be used for the different sounds as well as different therapy games. This site also has resources for literacy, language, articulation and communication.
Mommy Speech Therapy site provides free information and resources about communication development. This site also provides pictures paired with words organized by the target sounds. This is a useful source for speech therapists looking for free materials and for parents and family member's who would like to something to work on at home with their child.
Created by Caroline Bowen, this website is a great source for speech therapists working with students with phonological disorder. It provides free picture cards of minimal pairs (ex. cat and bat), pictures for vowels and vowel contrasts, and backward chaining information.