Home Depot doesn't seem to have 5-gang boxes - can I put a 3-gang and a 2-gang box together assuming that existing wiring reaches as required?
If HD is your only option then they can order it for you. Here is one -
If you put two together then the spacing will not be right for the cover plate to line up for all switches.
They do make modular ones so you can daisy chain several together, I have seen them a lowes but i can not find a link.
You may need to go to a dedicated electrical supply house.