Might be common knowledge, so forgive me if I'm just late to the game.
I've been on the hunt for a K focusing screen for my F4, but it seems they might be fairly rare. The few I've found on *b*y have been ridiculously overpriced. Cheapest I found was $150, most expensive was $250.
No thanks. You can find near mint copies of the whole camera in that price range.
I started doing some research and discovered that while the metal frame is different, the glass is identical in size to the screens for the F3.
So I paid $16 (shipping included) for an F3 K screen and swapped frames. Four very tiny screws and L-brackets are all that hold the screen in place.
It works great, including auto focus. I thought maybe the metering could have issues (since the center circle of the K screen is so much brighter), but I tested it in spot mode with both the K and the normal B screen and got identical results. I didn't test cw or matrix, but I assume they'll be fine, too. The negatives of the handful of test shots came out great.
Anyway, if you're willing to spend 10 minutes doing simple surgery, you can save a whole bunch of money.