If your child is age 0 to 3, she is eligible for a free evaluation and (usually free) services through your state's Early Intervention program. Find a list here. In addition to occupational therapy, your child may eligible to receive services through EI such as physical therapy, speech language therapy, and special education. When your child "ages out" of EI after her 3rd birthday, your EI service coordinator will help you transition to services supplied by your local school system.
Ask your local school district/school board for an evaluation by an OT if your child is over age three. This service may be handled by the school system's special education department. Because schools sometimes don't recognize the importance of sensory problems, it will help to notify them of any developmental delays or diagnoses your child has (such as ADHD, asthma, etc.) Should "related services" such as OT be recommended after evaluation, the school district is mandated to provide school-based therapy at no charge. Note that the administrator you deal with will probably be unaware of the staff or contract OT's individual training and area(s) of expertise. While there are many wonderful OTs working in the schools who have strong SI backgrounds, there are many others who are more expert at evaluating and treating difficulties with fine motor, gross motor, and visual-perceptual skills. Therefore, you will have to politely insist that the evaluator (and the treating therapist should OT be recommended) be willing and able to evaluate and treat sensory processing problems.
You always have the option to hire an OT privately. Many parents find they can easily get insurance coverage for the OT of their choice. Others find they must first ask the insurance provider if they have a pediatric OT on their list of plan-approved providers. (They probably don't, but if they do, call the OT to find out her specialty because it may well be that she does, for example, hospital-based orthopedics, which is unacceptable for your purposes.) You may choose to go the private OT route if the school does not have an OT available (in this case, they should foot the bill), you need help at home and in the community, you feel the mandate is inadequate (e.g., sessions are often for just 30 minutes including the time it takes to pick up and drop off your child to and from class), and other reasons.
Ask your pediatrician or local parent advocacy center for a referral.
The American Occupational Therapy Association may be able to help you find an OT in your area. The AOTA website also lists accredited university programs and provides links to state OT associations. The state contact person may be able to put you in touch with an OT specializing in sensory issues as well.
The Kid Foundation's SPD Network has a resource directory of OTs, physical therapists, speech language pathologists, and others.
To find an OT in Canada, try OT Works and click on "OT Finder."
Developmental Delay Resources, a nonprofit organization dedicated to helping children with developmental delays, sells a directory of OTs, other professionals, and organizations interested in sensory integration and related issues.
Western Psychological Services This web site lists some of the therapists in the United States and elsewhere who are certified to perform the Sensory Integration Praxis Test (SIPT).
Join an SI support group through Yahoo Groups (search for "sensory integration" to find appropriate groups) and ask members for recommendations for OTs in your area.