I agree that it better to replace the u-bolts, mounting bolts, spacers, and shackles. I would definitly replace BOTH springs so that they are a matched set. Try finding the parts locally because shipping on the springs will be kind of high. Try a local tractor or farm supply shop.
If you know someone with pnuematic tools you can have the job done in about 15 minutes (I know from experience.) Without air tools count on more, maybe an hour. I think that is why my friends keep dragging their broken stuff to my house, I seem to have all of the tools.