OpenLink Software does not supply the Informix native client. Instead, the Informix native client should ship with the standard Informix DBMS install media. At the time of writing, staff notes that the Informix native client appears on the Informix SDK CDs. However, packaging and marketing labels are subject to change.
If you do not have the Informix client install CD, you should visit the IBM Software Mall to see if the appropriate Informix client version is available for download.