It looks to us like King Arthur has cut its selection of mixers recently, but fear not, several other sites have what you're looking for. Amazon.com has them in two sizes, big and huge.
The Kenwood KM410 Chef Classic 5-Quart Stand Mixer has a 600-watt motor and can handle 3 pounds of bread flour, 6 pounds of cake mix, or 12 egg whites at one time.
The Kenwood KM800 Major Classic Stand Mixer has a 650-watt motor, a bowl capacity of 7 quarts, and can handle 3-1/3 pounds of bread flour, 10 pounds of cake mix, or 16 egg whites.
By contrast, KitchenAid's new Professional Stand Mixer has a 525-watt motor and 6-quart capacity, and, at the time of this writing, is priced in between the two Kenwood models.
Cooking.com, another of our retail partners, also sells the larger Kenwood and various KitchenAid mixers.