What pre-built computer from best buy Canada for under 1000 can run source engine games like half life 2 and TF2 and Portal at mid or max settings with no lag

Consider buying from iBuyPower or CyberPowerPC instead of brick-and-mortar shops, since they tend to have higher prices. Also, I don't think the correct term is lag, since that is the not affected by PC specs (except maybe your wireless router and receiver, if wireless), I think what you mean is 30+ FPS. In that case, check the video card, RAM and CPU. CPU should be at least dual-core with at least 2 GHz, 3-4 if its a single core. The GPU should be something like an NVIDIA 9000 series, or an ATI 4000 series.