You could also try Lowering your resolution, of course it takes away from the appearance and makes things look different but it boosts FPS. You can do so in video options ingame
You could also try to get an FPS config for CS:S, I know they can boost FPS a lot.
If you google it, there is a lot and they are all different for different types of computers so I couldnt just tell you a specific one, its whatever works for you.
Aswell as make sure all unneeded programs are off during play
And defragging your computer and the game itself never could hurt.