Patrick,
I usually make my own Ronchi gratings....
You can at a push use a piece of fine fly wire mesh...
Can't see any eBay suppliers but:
http://www.ronchiscreens.com/screens.php
and some background..
https://nicholoptical.files.wordpres...onchi-test.pdf
http://www.jeffbaldwin.org/ronchi.htm
Note: similar results are obtained when using a grating on a telescope while viewing a moderately bright star.