You might want to go to the Dexter website and download the service manual for light duty axles (http://www.dexteraxle.com/). Print it and keep it with your other manuals. There is a lot of good information in the manual that can give you some piece of mind and help you to maintain your axles, brakes, and hubs.
Most replacement springs (I've ever seen) come as a set of two or three leaves already assembled with poly bushings in the ends. When you replace that spring, remember that the spring shackle bolts do not turn, they are spline-fit into the shackle. So only turn the nut (but hold the bolt head with a wrench, anyway). If you reuse the nut, use a locking fluid to secure it (new nut would be preferred as many are interference-fit and they aren't as secure the second usage). If you do go with a higher load-rated spring, remember the axle and bearings and brakes are still sized for a lighter load, so don't exceed the GVWR for the trailer. My apologies if you already knew all this...
Steve