Where can I buy a mortar and shells?

The neighbour kid keeps leaving burning bags of dog s*** on my porch. This is really pissing me off and I want payback. The weather institute claims it's going to rain hard next week, so I've put together a great plan. I want to blast their roof off so that the f****** can just sit there and be soaking wet. I think it would be best to use a mortar to do this, but where do I get one of those?