The folks at Katzeye optics make replacement screens. Although these screens cost a fair bit more, there are versions with split prisms, which have really helped me with manual focus.
Katz Eye Focusing Screen for the Pentax K10D @ KatzEyeOptics.com
Like John said, handle the screen gingerly. When I cleaned mine, all I did was run it under distilled water and then carefully dry it with a hairdryer, all the while holding it by its tab with tweezers- I never physically touched it (yes, my fingers got pretty tired).
John