I used a 55gal plastic drum when I first started my mobil service over ten years ago. I got it for free :jump I just put it on it's side on a 2x4 stand in the back of my ranger. Tou can find special tanks for detailers at ronco-plastics.com
Make sure to use a 1" hose so you get good flow and if your pump has one of those glass filter on it remove it and use a screen one. If you want to set your pump on the ground when in use only make the hose long enough for this. The longer the hose the slower the flow.