The same goes for cable ties: you should get a small handful included with your case, but you can buy more for a pittance, and they're perfect for tidying up thickets of PSU cables.
Finally, get a little tube of thermal paste, which is a gloopy grey compound that facilitates heat transfer between the CPU and its cooler. A lot of coolers come with pre-applied thermal paste, but it’s good to have some extra to hand, and for future CPU or cooler upgrades and maintenance. The same goes for cable ties: you should get a small handful included with your case, but you can buy more for a pittance, and they're perfect for tidying up thickets of PSU cables.