K.A.
You can get replacement parts directly from Lego. It's especially helpful if it's a specialized part. You can buy big lots on ebay all the time and some people will even sell individual pieces, especially if it's a specialized part. If you have a Lego store near you they sell loose Legos in bulk.
Another thing to remember is you don't need to only build what the set comes as. When we get Lego sets the kids follow the instructions and build it as is but after that it all goes into the big dump bin of Legos and from there they build whatever their imagination comes up with. We keep the instructions and every once in a while they will go back and remake whatever it was sold as. There have only been a couple of sets that they put together and left. Those are the more expensive and elaborate ones that are more like a model kit. It took my son days to finish the big AT-AT from Star Wars. It sits on his shelf just like a model kit would.