[Les]

American Dj used to have a light called the "Groove Wheel" which had a built-in rotating oil-filled disc. It was sort of like what you describe. If you look around, you might be able to find something similar (all I'm finding are water effects though). Ebay might have a used one. I tried "Liquid Light Projector" and all I got was a small home use one. Your mileage may vary.