i am looking for something around the 100 gallon range and it does not have to be pretty
A google search for "100 gallon livestock watering trough" will get you onto some. Have seen these used.
A cheaper, and more local option is 2 of these http://www.lowes.com/pd_314139-61896-314139___
Good luck with build, post up some pics
You may not be able to use these. Would have to see what type of plastic it's made out of. It could possibly leach bpa or other chemicals into the water.