One option is a Rubbermaid stock tank. I believe they come up to 350 gallons, but I don't know if the dimensions are good for you. I got my 100 gallon tank at Tractor Supply. That's the biggest that would fit in my Prius.
Had a hard time finding a good horse trough one..& a large(deep in wide) enough one at hydro shop.
But I fell in love with the fiberglass tanks with window at aquatic ecosystems.
I found the one I wanted- 72 by 36 by 24 for $718 in my 2010 catalog, .... hopefully didn't go up too much