Have you tried searching at http://www.ebi.ac.uk/ (this is just one source - there are many)? Or a publication search could be very helpful in locating datasets of interest. Where the source data is located should be disclosed in any publication. If not, you may want to find another, or could consider contacting the authors for clarification.
There is a tool built-in under "Get Data" to search and retrieve data from EBI SRA. Import using the link for "send to Galaxy": https://wiki.galaxyproject.org/Support#Get_Data:__EBI-SRA
That said, any data can be loaded if in the correct format for the target tools. If there is a URL retrieval method disclosed by the source, use the Upload tool. If you have the data already downloaded locally (or this is the only way to retrieve it, use FTP to ge the data into Galaxy.
How to do these operations is in the same section as the other data loading help in the link above. But please let us know if you need any clarification or run into problems.
Thanks, Jen, Galaxy team