ACI has the three subwoofer drivers built to spec by a great U.S. OEM located in California. They've been doing a great job of supplying us for about 10 years.
The rest of the drivers we currently use are sourced from Vifa, Seas, Morel, and ScanSpeak. All make excellent product with superb QC.
Some companies claim that they are using proprietary drivers when in fact they are off the shelf units. Ultimately it is the designer and the implementation thay really count.