Lee filters stocks accurate ND acetates in densities of 0.15, 0.3, 0.6, 0.9, and 1.2D. My densitometer measurement of these has shown that they're within 5 or 10% of the stated density, even though they're only meant as lighting gels.(posted 8262 days ago)
If you e-mail, phone, or write to Lee, they might send you a free sample swatch of their lighting filters. The swatch book contains 75 x 25 mm samples of all those ND filters, plus dozens more coloured filter and scrim samples. Each coloured filter comes with a transmission curve as well! What more could you want for free?
BTW Ryuji; a piece of clear acetate film has a density greater than 0.05D, you won't easily get steps that small, and anyway, that sort of accuracy is of little practical use in photography.