The two most common download sites are Avsim.com and FlightSim.com. Both require membership, although they are totally free to join. Avsim.com gives you guaranteed access to lots of downloads 24/7, although FlightSim.com uses nodes, so logging in might be a problem. You might find that the site is operating to maximum capacity and that all nodes are full, so don't rely on this website only.
Alternatively, check out the likes of IFDG.net - IFDG makes some good A320 family aircraft and ProjectOpenSky.com - they make some excellent 737, 747, 757 and 767 aircraft.
All of the websites mentioned are for free downloads. If you are willing to purchase professional-quality aircraft, head over to SimMarket.com.