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Microscope objectives, in general, are designed to work with 0.17 mm coverslips. You will get the best image quality using coverslips of exactly this thickness. Using coverslips of the wrong thickness will induce spherical aberration.
There are now a number of sources for high precision (0.17 +/- 0.005 mm) coverslips:
Suppliers (these don't sell direct, but might be useful for finding other distributors: