Depending on what you wanted to make you would at the minimum need if you don't want to be searching around you can get just about everything on amazon.com
1.power source, 18650s are the most common for simple box mods
2.battery sled with terminals to put the batteries in since you don't want them just sitting in the box
3.fire switch,a momentary button that you can wire in which will fire the mod
4. Wire to connect everything.i usually for mech mods have 14awg wire
5.510 connection this is what the atomizer will connect to fat daddy vapes as well as varitube has good ones
6.voltage display screen to see what your box is firing at.
7.some sort of chip if you are going with a mech mod a mosfet is perfect and should protect you and your box
Like I said just about everything you can find on Amazon if you are looking for convienence