Any automotive parts store, Tractor Supply, Home improvement stores usually have all the parts you will need.
You can put in a single outlet or buy a "triple" outlet for a little more either way, it isn't much more than connecting the power lead and making a good ground attachment.
Make sure you use heavy enough wire gauge to support whatever load may go through and that the outlet is rated for it. If it doesn't have its own fuse protection- add an in line fuse.
Your choice on wiring directly to battery or trying to tie into an appropriate circuit that is on the key switch.
In general you can go as simple or fancy as you want.