If you want the source to travel at a speed like the speed of sound in the air, you would have difficulty accelerating this source since you are dealing with neutral atoms (I think). One can maybe overcome this by accelerating ions, letting them collide with a certain material so they pick up a certain charge or a charge gets stripped off. In the end very fast-moving atoms come out of the material. These atoms can then be used in the resonant scattering experiment.