Power Comes into Light fixture from Panel, connected to switch how to wire always on outlet in
In order to install a receptacle that is not switch it must come from a location which has a source conductor, neutral and ground. If the power source is coming into a light fixture first then that would be the place to tap it. However if the switch was installed under the new NEC requirements, then it should also have a neutral and you can tap the conductors from there. Look to your feed going to the switch and see if it contains 4 conductors a source, switch leg, neutral and ground. I not you can also replace the feed from the light fixture to the switch with the correct cable. 

Good luck