I am in BKK now, having purchased an I fare TPE-BKK-TPE just a couple of weeks ago. This, however, was part of an Early Bird special that they had running until the middle of June. Lots of city pairs were available during that special. I also booked an I fare for TPE-LAX-TPE for August just recently (it wasn't any bargain, but still cheaper than a D).

The I fares are out there. As others as said, you may need to call CX directly since they aren't always obvious on the website. Otherwise, keep an eye out for Early Bird specials, weekend specials, or similar deals, many of which feature I fares.