Broken leafs are fairly common. Most DOT's require at least 75% of the leafs to be unbroken. Look in the yellow pages of the nearest big town/city phone book under "Springs, Automotive Sales & Service" and give several of the listings a call. They'll have replacement stock spring packs in stock or can tailor custom made springs to Your requirements. For an additional fee of course.
Going to a full air ride rear suspension would be WAY more $$$$$$. Would ride much better but probably not worth the expense.