I bought mine ( a 50*50mm , 600 g/mm) from Richardson Grating Labs USA (now
Spectra-Physics), and constructed a very servicable spectrometer using it. A
sine arm controls the inclination of the Littrow-mounted grating, using a
Mitutoyo digital micrometer.
Digital readout is closely linear to wavelength.
contact:-
SPECTRA-PHYSInCS
705 St. Paul Street
Rochester, New York 14605 USA
tel: 585/248-4234; fax: 585/262-3206
web site: http://www.spectra-physics.com/gratings