I normally go to the local Lowes. I have learned to get everything that I might possibly need, if I'm not sure of the size of whatever I need, get both sizes, get extra this and that ( to a reasonable point), and when I'm done return what I didn't need, in unopened undamaged packaging of course.
I commuted to work at a power plant for several years with an electrical assistant superintendent. He advised ordering four of everything you needed: one for the warehouse people to lose, one for the craft people to tear up installing, one for the inspectors to tear up testing, and one for everyone to get it right.