best free online open source for DEM / LIDAR / contours / topo / orthoimagery etc for site development jobs?

[Emil's Dad]

Hi folks -

does anyone have any recommendations on best sources for DEM / LIDAR etc? I have tried USGS Earth Explorer, OpenTopography, Google Earth Pro, Web Soil Survey, and local county Assessor's offices (for GIS / property data), etc - also one of my international colleagues likes Global Mapper - but is subscription based ... my hope is to find some govt / tax-funded agency that provides reliable data (at least across the US, but preferably globally) to be able to build realistic sites (though not total station topo survey quality) in Civil3D ... any suggestions would be appreciated ...

thanks & Happy Holidays

* also any recs on sites that clearly explain optimum use of the various horizontal & vertical datums...