I've often seen photographers ask in forums where to buy fake ice. There are other options of course; but one such place is
Trenglove Studios in New York. Larger pieces for closeup shots are $40 for acrylic or $35 for solid glass. Bulk "display ice" can be purchased for $15 for a dozen. If you're in NY, you can rent your fake ice instead of buying it. They also have fake splashes, liquid spills, etc., even fake bubbles for that extra clean shot.