Depends what you want tank wise and where you want to put it as you could either diy one yourself or get even get one custom made. Can't remember the chaps name but he is able to fabricate tanks for vans or static systems to your spec.
I remember seeing on a facebook a custom made insulated static 1000 L ro/di system for a chap to fit outside where space was limited down the side of his house.
I also know the chap built a custom static 3000 L tank setup for Lance at sunshine windows recently.