did you try the yellow pages? and maybe you should specify which kind of treatment you have in mind. If biological, these units are only mobile with regard to the transportation to the place of work. once filled with the sewage you can hardly move them. These systems may be called "interim or provisional or temporary" treatment plants.
We are going to get a small mobile and compact system that can be carted to any location and operated. The system generates high levels of supersaturated oxygen that breaks down the sewage in matter of hours.
If you require more details please send me an email at advenvtech@gmail.com. I am based out of Mumbai.
I suggest you to see at www.eecusa.com. This is a manufacturer from USA. They produce MBBR container type of WWTP, with standard capacity up to 350 m3/day and more. I think they will have what you need.