I got mine given to me from the hospital physio, pretty much the same as the one in your link (it was a cheaper looking version though). I am guessing that the ones you have seen at the chemist going around the stomach are lumbar (low back) support ones, not targeted specifically at the SIJ.
Hopefully the Physio gave you some good tips, but other things that helped me:
1. Avoid lifting!!!!! This was the absolute hardest (DS is 2, and I have a manual job), but was the difference between being able to walk at the end of the day, and being curled up in tears!
2. Avoid asymmetric activities, ie no standing on one leg / weight mostly on one side. I even sat down to put pants on.
3. Avoid lifting!!!
4. I slept on my side with a pillow between my legs
5. Avoid lifting!!!
6. Try and minimize amount of time walking/standing
7. Avoid lifting!!!
8. Core stability exercises (Physio should be able to give you the ones suited to you)
9. Avoid lifting!!!