Hi Peter,
two options come to mind. You can source a Ronchi Screen from RonchiScreens.com and make up your own test eyepiece by cutting the screen to size to fit into the shell of some old eyepiece you have deemed useless/unsatisfactory,(of course discarding the old lens set first). Or secondly google up: Schmidling Productions, Inc. for their Ronchi 133lpi Easy Tester eyepiece.
