Depends on what do you mean by “high resolution”. There are many free sources like SRTM and ASTER satellite data sets that have resolution of 30m/pixel for the entire world. But I think that the resolution is even better for locations within the US. Check also USGS website for that.
I mean the highest resolution possible. I’m sure there’s the entire world in 30m per pixel, but I’ve seen people talking of 1m per pixel or even 50cm per pixel for some places in the UK, I’m just not sure where to get those or even how to convert them to get the full resolution in some useable format for ue4. Maybe there’s. Even better res than that.