I think you’re going to struggle to find something that can carry 200lb and be collapsible. Most carts seem to either be for much less than that, or much more. I found several that looked a bit like what you may want by searching for ‘vendor cart’.

You may well be better off building/comissioning something to your spec though as a lot of the bigger carts are designed to be food stalls when stationary, so they’re probably unnecesarily heavy. I think you’d be able to make something along the lines of what you wanted with parts from your local DIY store.