Light posts for decks can be purchased in a number of designs at a range of price points or can be custom built using low voltage landscape lights parts and a little ingenuity.  Because it is low voltage anyone can perform the installation, an electrician is not required.  Off the shelf low voltage lights usually are side-mounting units that require you to create a cavity in the posts to distribute the cable.  Many builders install a sleeve around the post to conceal the wires as they are directed under the deck frame.  Others use a router to create a channel in the top rail and pass the wiring between posts in the rail, which is concealed by the top cap.

Some light posts are designed to fit onto the top of a rail post like a post cap.  There are even solar powered battery operated options that eliminate the need for wiring.  Custom light posts are created by cannibalizing parts from low voltage landscape lights and attaching them on top of the rail posts.  You can then build a sleeve out of 1x6 cedar fence boards or a composite post cover to conceal the wiring and add a decorative opening with a router at the top to release the light.  Finally a removable post cap should be installed above the light for protection and access to the light bulb.