Who should test my son for dyslexia?

I am 98% sure our 8 year old son has dyslexia. Should we see an independent certified dyslexia screener for the test (she can see him and start the process next week) or should I go through Children's Hospital and work with the psychologist who ures me the test cannot even be given for another 5 weeks? My gut is saying screener, but we are afraid there is some down side to that route that we are not aware of. Thank you!