We used the school resources, but that was appropriate for us because our state and district have the policies, resources, and experience for dealing with gifted children. This had two advantages... not only was it free for us, but the school had no doubts about the validity of the results.

So I guess the answer here depends on where you live. If the school has the appropriate resources, that's your best bet. But if they don't, you should definitely go and find them on your own.