If you can't find these used, GM actually sells a bunch of these as kits. Without AC they are around $500 but that includes alternator, power steering pump, and all the brackets, tensioners, pulleys, and bolts. I am not sure it includes the water pump, and the AC kit is extra.
Here is a link to the Chevy website. To get the best price, just google the part number and go to the shopping tab.