Any "green" builders shops around you? We have some - they sell stuff left over from construction, orders that were cancelled, etc You can usually get some really good deals at those types of places.
I would contact a plumber's supply place. If you know the name brand of the toilet (perhaps take a picture of what it look like if you go there in person.)
Did you just break the lid on top of the tank?? You might be able to get a new lid and tank too.