You could either get a sling that is capable of being used in the back carry position, or you could get a backpack. There are really nice hiking type ones that can hold a larger child. They're somewhat bulky, but extremely comfortable to wear, we had one that we used with our daughter until she was about 2. When we went places like the farmers market where it wasn't fun to push a stroller, but we wanted her contained. I had a Kangaroo Korner fleece pouch sling that I used with her until she was close to 2, but I didn't use it too much in the back carry position. For me it was more comfortable to use in the hip carry, but I know it could have been used in the back too.
Hope that helps.