Mossberg accessories are everywhere. Many parts/accessories are interchangeable between 500, 590, 835, etc. I suggest brownells, midway, or find online and purchase at your LGS. As far as what to put on it....make it your own....a million suggestions but you have to decide what you think you need/will use. I would suggest putting a few hundred rounds through it first..might not need anything except a light (if sd/hd). Good luck and have fun!
“The beauty of the Second Amendment is that it will not be needed until they try to take it.”
― Thomas Jefferson
“The constitution shall never be construed...to prevent the people of the United States who are peaceable citizens from keeping their own arms.”
― Alexander Hamilton