The statement on the bottle of T-IV means that the new fluid can be used in older transmissions that used Dexron III, but it does not mean that you can use Dexron III in the newer transmissions that call for T-IV.
You can get transmission fluid that meets T-IV specs at Autozone and other outlets. Castrol makes it as do some other now, I think, but it costs about the same as getting it from a dealer. The advantage is that Autozone etc are open later at night, on weekends and may be closer to you.