Less cost than Eaton, but still good selection and quality are leafs made by Stanley Springs in Harrisburg Pa. Contact them for a local supplier. Otherwise, see what shipping is from Stengel Bros.
http://www.stengelbros.net/Leaf-Springs_c_177.htmlAt the bottom of the page is a link to listings of springs available by application.
Having gone the helper spring route as a greenhorn, allow me to suggest ditching them at the first opportunity. They really are best for situations where there is extra weight being carried in the rear. They make the rear stiffer (in the upward direction) and in doing so make the car more likely to fishtail and spin.* This is worse at higher speeds (like a highway to highway interchange) and slicker conditions. Don't ask me how I learned this before I understood this...
You can get a little more ht in the rear by cranking up the front. Factory spec on front ride ht for Dart/Valiants is about 2 1/8" IIRC. That's from the bottom of the t-bar anchor to the lower ball joint. If you change the ride ht, then reset camber, caster, and toe in that order.
Leaf and coil springs do lose rate and hieght after 75 - 100,000 miles. Quicker if abuse (overloaded, raced with stickies) and longer if only driven on smooth roads by super smooth driver weighing 95 pounds. Also the interleafs wear out, but those and the clamps (bands) can be replaced.
*edit: With extra weight on the rear, the extra spring rate won't tend to cause fish tailing. One method trucks and utility vehicles use to get best of both worlds is have a bottom leaf whose ends hang in the air until there's a bunch of weight in the bed and the leaf pack flattens enough to touch.