There are several commercial reptile cages on the market... something like
Vision Cages are large enough and quite nice. They are stackable, and have slots for lighting.
Boaphiles are also pretty neat, and can be customized with lighting fixtures in them. No commercial cage option is going to be cheap though. You'd be looking at probably at least 300 dollars for something that size.
I build all my large cages out of plywood with Plexiglas doors. It's a lot cheaper - I built three 8x3 foot cages for the price of a single manufactured cage - but it also takes some carpentry skill, tools, and significant effort.
