spdfoundation.net is another good site to check out.
I've been a pediatric OT for 10 years, and have worked with LOTS of pediatric OT's in practice who are not necessarily "certified" in SI but are very skilled and experienced at treating sensory-related developmental concerns in children. The certification process is 4 week-long classes and costs several thousand dollars to complete, and many OTs decide NOT to be certified because the certification focuses primarily on teaching how to administer and score the SIPT (Sensory Integration and Praxis Test), which is a test that has a very limited sub-group of children on which it can be used. The SIPT also takes a long time to administer and gives information that can be gleaned from skilled clinical observations and other standardized testing for gross motor, fine/visual motor and perceptual skills. Very few SI certified therapists use the SIPT in their clinics due to those issues.
Also- there used to be 2 organizations that certified therapists in SI, but one of them is now defunct (Sensory Integration International- SII). Western Psychological Services (WPS) is the only organization that now certifies people, and it is a cash-cow for them due to how much people pay to be certified. SII and WPS had some legal battles about who had the "rights" from the original creator of the theory of SI, and WPS won evidently. It was a messy legal thing.
I think the spd foundation website I listed above has a list of questions you can ask any OT that you would interview, to determine if they are skilled in addressing sensory issues with kids. You might pass up a really great therapist if you only look at those who are certified.
Just my two cents. but I should add for the record that I'm NOT certified so maybe I have a biased opinion.

Hope this helps!