Originally posted by dlh thanks for the tip; after a bit of searching, I ordered a couple of screens from:
Focusing Screen in Taiwan R.O.C.; two hundred bucks incl. shipping.
Installed the first in the K-50 this morning. Installation was a snap, as long as one is careful to observe which side is up and which side the tab is on, prior to fully removing the old screen. Nowhere near as complicated as, say, rebuilding a Rochester Quadrajet. Testing for front-to-back focus showed no adjustment needed - picture of a typesize guage below, focus was on the six-inch point. There were two things that bothered me - first, "focusingscreen.com" is in Taiwan, and their English needs a lot of work - the instructions are pretty much unintelligible. Secondly, they say they need ten-day turnaround because the screens are "custom-made to order". What I got were screens in standard Canon packaging; they had sticky labels attached showing which Pentax camera they were for, but either they're not "custom made" or they're trademark knock-offs. My guess is the former, they're probably drop-shipping from a Canon distributor or something. The difference is that if you know what to look for, you can order the same thing off Canon's website for a third the cost.
However, I didn't know what I was looking for, and it was worth the extra cost not to have to think about it. Summation: I'm happy, it's a terrific improvement over the original screen.
I'm a mostly-manual guy, generally put up with live view and autofocus but I hate 'em. So the improved split-image prismatic or whatever it is, is an enormous relief, improves the utility of the camera many multiples.