I called the rehab departments of the hospitals in the area - I live in a large metro area, so the name of the person who coordinates the observation/volunteer experiences was actually listed in most cases, just check their website. I just called them, and in some cases left an email too, and continued to follow up until I got a hold of someone. They get a lot of requests for career observation hours, so don't be surprised if you have to be a little persistent - and be all right with taking as many hours as they are able to give. I ended up going to a couple of different places in order to get all of my hours, since most places could only give me two full days' worth, or 10 hours, etc. However, in the long run, it's better if you spread out your experiences, so you can see different stuff.
I also happen to know an OT, whose kids I used to babysit, who works in a private pediatrics clinic, so I did a bunch of my hours with her, and then the rest through the hospital system in my area.