You can use the SpatialReferenceEnvironmentClass. Cast it to an ISpatialReferenceFactory and use the "CreateGeographicCoordinateSystem" method. For ex:
ISpatialReferenceFactory spatial_reference_factory = new SpatialReferenceEnvironmentClass();
int wgs_84 = (int)esriSRGeoCSType.esriSRGeoCS_WGS1984;
IGeographicCoordinateSystem geo = spatial_reference_factory.CreateGeographicCoordinateSystem(wgs_84);
ISpatialReference spatial_reference = geo as ISpatialReference;
// you can set your spatial reference domain / etc... here if you'd like
spatial_reference.SetDomain(-180, 180, -90, 90);
Then you can use this spatial reference when you call Project.