I thin it will probably be cheaper to have a pad poured in place than to get a precast 16x10 delivered and put in place. Anything that big precast will have reinforcements (rebar) inside, and will be equally difficult to break up, and will have to be delivered by crane truck.
You could try asking local concrete places, the sorts that sell septic tanks and so forth. You might also ask at any local boat dock places for leads; one not uncommon use for such a precast pad would be a pier or dock, where it's not practical to just pour something in forms.