Tell them that if you don't have the clamps in your posession by next weekend, that you're gonna send the entire exhaust back and will want a full refund of your money + shipping!! I'm sure that they have sold plenty of these exhausts and since they have all the part numbers that they need, there's no reason that they shouldn't have an answer by now.

Is the exhaust a GM product? If so, send me a PM with the part number and i'll try to contact a dealership and see if i can get a part number for the clamps.