Whether you're looking for data to reuse or integrate with your research, or trying to find somewhere to deposit your own data, a relevant data repository (also known as an archive or data centre) is a good place to start.
If you want to use someone else's data, you should always:
You should always credit the original author of any data you use. There are, as yet, few standard citation conventions for data. However, the following data citation guidance, taken from the Digital Curation Centre's 'How to Cite Datasets and Link to Publications' will help:
If there isn't a suitable specialised repository, you could try one or more of the following more general options: