Measuring eyepiece: grid replacement?

[nielsgeode]

I can purchase two Leitz Wetzlar Periplan GF 10x M eyepieces from a friend. I'd like to use these in my Leica Stereozoom to estimate crystal sizes.

The current grid is a large square with a smaller square in one of the corners of that large square. Since the square size is much too big (even the small one), I'd like something with smaller grids or lines closer together.

I did an Ebay search and found plenty of them, most are 19 - 20 mm in diameter, but my grid (round piece of glass which you can remove from the inside of the eyepiece) measures 17.5mm. How/where can I get these grids in that size?