THE band you must check out when it comes to post-rock is Godspeed You! Black Emperor.
Others groups I would recommend off the top of my head are Sigur Rós (most of their stuff isn't instrumental, though) and MONO (you can buy their stuff on Bandcamp so you might've ran into them). Another well-known group is Mogwai, but I've never actually listened to them myself. I guess the later stuff by Talk Talk is considered post-rock as well. I HIGHLY recommend checking out Spirit of Eden and Laughing Stock, both are masterpieces and widely considered to be some of the best albums ever made.
If you don't mind a heavier sound, on the metal side of things group called Isis is well worth checking out. Oceanic and Panopticon are both incredible albums. Of those two, the latter is probably the easier one to get into at first, but I personally find myself preferring Oceanic after hearing it enough times to learn to appreciate it.
Edited by TJ Elite - 3/9/14 at 8:38am