Don't think you need sulphur "crystals". I'm not sure if I have ever seen them sold, except in rock shops.
One usually finds sulphur as a soil amendment as "ground sulphur" or "sulphur flakes". "Sulphur powder" is fine but usually more expensive as it can sold as a fungicide. Work time is months to years for the first two I mention. I have never used sulphur powder, but due to its enormous surface area for reaction with the soil (compared to the others), it should be much faster.
