You can get the springs at Home Depot or Lowe's. They come in a pack of, I think, 84 assorted springs for about 4.00. There are 4 of the medium springs in that box. See this
thread on the PS forum for more info and a photo of the package:
Medium Spring
It really shouldn't require regular maintenance outside of just cleaning it out occasionally. Dust, dirt, and debris can work its way in. You just need to take the assembly out, break it down into its parts (there are only a few), and wipe them down with a napkin or cloth.
And finally, I've wondered the same thing about the need to replace the brass screw. Since these don't seem to oxidize or pit like the previous ones, I'm not sure what exactly will ever wear out. Maybe just check it every few months or so to see if it's wearing down... not sure.
Oh.. saw your last question: if I notice a drop in power and everything else is fine, I wipe down all of my
threads and clean my adapter. Steve recommends soaking the adapter in vinegar occasionally. I've done that a few times, and it does seem to help get the old
juice off and out of the threads. But mostly, I just wipe the adapter off with a napkin.