OpenLink Software does not supply the Ingres native client. Instead, the Ingres native client should ship with the standard Ingres DBMS install media. At the time of writing, staff notes that the Ingres native client appears on the same CDs that contain the full DBMS. However, packaging and marketing labels are subject to change.
If you do not have the Ingres installation CDs, you should visit the CA site to see if the appropriate Ingres client version is available for download.
CA has product FTP sites.
However, these sites are more apt to contain patches to specific products than useable, standalone products.