pcpartpicker.com
you don't need to spend $1000 on a computer if you just want to play league. You can play league on just about anything, it doesn't require much in the way of hardware.
But if you want a good build for around 1k, something along the lines of this works
http://pcpartpicker.com/list/gqjJM8
factor in about $100-130 for windows. you can go cheaper on pretty much anything there if you want, like you can can get a smaller hard drive and no ssd if you wanted to, you can get a cheaper graphics card like a gtx 960, you could go for a cheaper case like an NZXT Source 210, you could get a semi-modular and lower watt power supply, etc.
again, this is completely overkill to just play league though.