Rear Springs - Which Ones Should I Buy?

The possibility of finding a local spring shop was a good suggestion. There are a handful of spring manufacturers in the US and a local shop will be a retailer for one or more. This will save on shipping. Triangle Spring is one and Stanley Parts out of Harrisburg Pa is another. Stanley has many application specific springs for - just like Chrysler did originally. You can contact them for a seller/installer near you.

Or if you have to, these folks will ship them (from Allentown Pa)
http://www.stengelbros.net/Dodge-Lancer-Dart-and-Swinger-Leaf-Springs_c_469.html

Leaf and coil springs do eventually lose some spring, usually 70-100k miles especially if abused by drag racing or overloading. I do not recommend increasing rear ride hieght or spring rate over what the car came with unless there is specific reason to do so. Doing so can make the car tail happy and this will show up at the worst possible situation, such as downhill on a curve in the rain. If you carrying extra weight, then the extra initial hieght makes sense. If you want a stiffer, more sporty suspension, then also stiffen the front up by adding anti-sway bar (if not one there already) and possibly larger torsion bars.

My guess is 2-4 hours of shop time depending on how rusty and difficult it is. As MrJLR mentioned, parts cost includes the little things. Shop time around here ranges from 55 to over 100 an hour depending on where you go and type of operation.