if you have many distributed areas, you can try GETASSE or CDEM (both AutoDownload) and compare if they produce less artefacts than ACE30. But they are also of low resolution compared to SRTM or ASTER.
If you decide to manually dowonload DEMs for terrain correction, you just have to make sure that these are projected in WGS84 and stored as GeoTiff. Please have a look at the help pages regarding this point
