There is that, but it's on a random basis, you flag the truck down when you see it. There's also a certain ribbon you can tie up high to let them know you want them to stop, but you need to be there, with lots of cash. You have to have a tank or container of some sort, and I doubt they're gonna wanna sit there filling up small jugs - but of course, if you need that, enough money talks loudly.
Having suggested two possible options, I'll still tell you, bring what you need!
Never count on buying what you need on-playa. Though there are exceptions, they aren't always reliable and that just isn't generally the way the whole thing works.
GreyCoyote: "At this rate it wont be long before he is Admiral Fukkit."