The Beattie IntenScreen Plus Is reputably the best focusing screen available for astrophotography. It is slightly brighter than the Olympus 1-8 but far darker than the Olympus 1-12 (that comes with the camera).
If you don't plan on ever getting into knife edge focusing, this is the screen to get.
The Beattie new, costs about $84. I will sell for $43.00 plus Shipping. Airmail $4.00 Surface $2.75