How to get WKT for various Spatial Reference Systems?

GDAL has methods for that. From http://www.gdal.org/classOGRSpatialReference.html: "contains methods for converting between this object organization and well known text (WKT) format".

ExportToWkt is probably what you need

"Convert this SRS into WKT format. Note that the returned WKT string should be freed with OGRFree() or CPLFree() when no longer needed. It is the responsibility of the caller. This method is the same as the C function OSRExportToWkt().

http://www.gdal.org/classOGRSpatialReference.html#a596b8e527d66a5054408159f0006fdac

There is also a GDAL utility program http://www.gdal.org/gdalsrsinfo.html

Usage example:

gdalsrsinfo  -o wkt epsg:4326

GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]]

I usually look at http://epsg.io or http://spatialreference.org for all my WKT needs.