Check out overclockers.co.uk I built my whole system with stuff brought from there, really good service. As for recommendations, there's quite a range of cards out there, what's your budget? For CS and L4D, you'll be fine geting something like a ati 4830/4850 (maybe even older 47xx ones) series or nvidias 8800/9600 series. You can find these quite cheap second hand on eBay too. But don't expect these to run recent games on full settings, you'll need to spend a bit more for that