Arround here I saw a couple stake beds with tanks on them that can also be used for spraying chems. on commercial properties. I also saw someone using an old fire pumper. I ws in AZ last week and saw a ton of trucks spraying. Thay all looked like the small gas tankers but for water only.
I thought the old fire truck was a good idea. Thay prolly picked it up at an auction or a junkyard real cheep and fixed it up. thay are made to hold a LOT of water w/o having to constantly re-fill or stress the truck much.
It all depends on how much you are willing to spend. and how much weight you can carry. Water is quite heavy.