If you credit the source, it's not plagiarism. It might still be copyright infringement though, and probably is in the case of Google Maps (Which is why OpenStreetMap have very strict and often repeated "Don't Trace Google Maps!" warnings for its users). Look for a public domain source, like Natural Earth, or one available under an open license like OpenStreetMap or Wikipedia (in which case make sure the license is compatible with the particular use you are making)

National mapping agencies (NRCan, Ordnace Survey, Swistopo, USGS, etc) often release some data under open licenses, and less frequently in the public domain.