Science Etiquette Question: Using Other People's Data

I realise this is probably not the best place to ask, but I don't know of any better (and I should really stop procrastinating on finding out.)

So, I got the peer review back from the first year paper that's (hopefully) going to become an actual paper published by my university. The big issue that was pointed out in the peer review was that, because me and the rest of the group working on this project were doing this with limited pre-research or oversight, we measured an interspecific thing intraspecifically. So instead of having a respectable ~30 data points, I have... 4. The peer reviewer suggested supplementing this sad amount of data with data from other research.

And I was wondering what etiquette was for using other people's data? I know you should make it very clear that it is not your data, and that is [specific other people]'s data, and cite them clearly. But should you ask for permission first? And if so, is there a polite(r) way of asking that permission.