I'd like to construct a custom reservoir but I'm not sure which material to create the inner lining from. I'm referring to the plastic that is used in a Flexitank.
I remember seeing a bunch of things like this at farms, only larger. Like huge bags of water almost, I'm pretty sure those were polyethylene like Mikell said. If you are going the alibaba route look for polyethylene fish tanks, those might be cheaper than something for "farming".