When purchasing ear defenders look at what decibel rating they're supposed to cut out and go for the highest decibel reduction. Even then you may have to reduce your expectations somewhat if you're highly sensitive to noise. Other people assume that they block everything out but that is not the case at all. In my experience all they do is dampen the noise slightly.
I find I get better noise reduction results from wearing both earplugs and ear defenders together. However it then becomes impossible to walk around without being overwhelmed by the vibrations from every footstep and the overwhelming amplified noise from within my own body. That method is unlikely to help in a supermarket environment if you are moving around.
You could also try noise cancelling headphones, which may have some cancelling effect on the trolley noise. However the best ones, such as Sony and Bose, are very expensive. Again don't expect them to work miracles, you will likely still be able to hear the noise just at a more tolerable level.
It's really just trial and error to see what works for you I'm afraid. What works for one autistic person may not help another.
