Finding research data can be difficult. There isn't yet a good way to search across repositories for a particular dataset or data on a specific topic. To find research data, you need to have a pretty good idea of where the data are likely to be. Consider the following when searching for research data:
Did you find out about the data through an associated output (e.g. journal article, conference paper)?
Are the data part of a large, ongoing project?
Are you looking for research data on a particular topic?