A great place to find some really good post-rock, experimental, ambient, drone, electronic, math-rock type stuff is from the Lost Children Net Label. And it’s a place where you can download the music for free as they care more about good music being heard than making a profit (and anyway do any record labels make much of a profit nowadays?).
There is over seventy albums available so if you don’t know where to start then I would suggest starting at The Silent Ballet compilations from which I’ve found many things to get further into such as French Teen Idol, Amberhaze, Orange Crush and Ben Frost to name a few.