large shower basin

I can't find large enough shower basins at Lowes/Home Depot. I'm looking for one about 7'x5'. Does anyone know where I can find a large shower basin or a custom shower basin maker (if such even exists)?
I understand how to build a basin "from scratch" through layering with mesh & concrete but it looks like a remarkable amount of work.

Try looking for 'roll-in' shower basins.

still looking

Wow, it's been over a year now and I still don't have a shower basin in the spare room. I found a place on the internet where they will build and deliver a customer shower basin but it's about $1K. Does anyone have any recommendations on a contractor who builds a shower foundation and tiles it? Thought I'd give this another shot before buying a tile saw.