benjojo does some clever data analysis for Picking uncontested private IP subnets with usage data.
He not only gathers the usage data of the various subnets, but also provides a nice tool to get a /24 subnet from the list that has no known users.

And to top it off the article also provides the underlying data.
A Zip file with the /24 subnets with number of known users (local copy).
And the distilled list of subnets that have no known users (local copy).