I think Kisers are the commercial industry standard. They come with a 350 or 500 gallon tank.
However, I'll caution you that 500 gallons is not going to keep an outdoor arena non-dusty on a hot and/or windy day unless you do it at least twice a day and the arena is no bigger than about 100 x 200. And that amount of water will do little for improving the shear properties (stability) of sand, if that is something you need.
Our local dressage club just had a licensed show at a new facility and I was helping the tractor man during lunch break by taking down the dressage fence so he could drag and water with a Kiser. Saw first-hand how slowly the water trickles out (needed to drive around many more times for watering purposes than the actual dragging). This was on a Sunday and despite it having rained hard on Friday morning and them watering it twice on Saturday and on Sunday morning, within 45 minutes of lunch break the arena was again like a dust bowl. There was also a problem since we were using 2 show rings plus 2 warm-ups that there was not time to refill the tank to do all of the other rings.
If you are speaking of an indoor, I highly recommend an overhead sprinker instead. I have a 70 x 200 indoor with sand/rubber footing and 250 gallons per day keeps it a nice cushy, but firm surface. It takes 2 minutes for this to be applied from a 350 gallon tank and then the well pump can slowly refill it over the next 24 hours.
If you have an outdoor, I think the best option for is a tank truck with a fire hose or high flow booms. Some people have outdoor sprinklers but they are tricky to place so coverage is even without mud spots. Something you could do to get an idea of the effect of a certain number of gallons and how long it lasts, before you invest in a pricier drag that doesn't do what you want, is to call your local volunteer fire department. They will probably be happy to come out and use it as a training opportunity for the rookie to handle the hose for a $50 donation. They might even know where you can buy an old tank truck.