The easiest solution would be to ad a switched cord like this one
It has a long cord so you can position it where it is convenient for you.
You plug the plug end of the cord into the wall and the lamp plug into the plug end and run cord with the switch to the area of your choosing.
The one liked to is dimmable so you can get a dimmable bulb or you can get a non dimmable one as well. I have seen them in the extension cord area of The blue big box home improvement store. I have two.
There is also this switch that can be added to your lamps existing cord. This requires you to cut one leads of the cord,( Make sure to cut the hot lead: the one that is not ribbed.) Strip the insulation back and connect it to two screws inside the switch.
Of coarse you would do this with lamp not plugged into the wall.