rhizobium are a family of bacteria (friendly types). They help plants fix nitrogen from soil/air and are naturally present in root nodules of leguminous plants. The culture is basically gel with a concentrated colony of the bacteria, but i dont think its available anywhere...I think it is to be applied when making compost- add it to compost heap i suppose?
But why not just get a good quality organic manure with nitrogen fixing bacteria in it? I use sunrich