I buy mine either at a Nike Outlet or I will wait to Dick's has a sale and buy 2-3 at a time.
You may want to visit a running shop to get help with a shoe that works for the shape of your foot and your running motion even if you don't buy shoes from them. I used to run heavily (200+ miles a month) and I wouldn't wear anything but Nike's. Not that their isn't other quality shoe makers out there, they just fit my foot perfectly. I have narrow feet with high arches and I tried several before determining that Nike's just fit better.
Also, now that I'm older (run less) and have more weight, etc, I've gone to Nike free run shoes. I used to have more hip/knee pain. Sine the switch, it's gone away and I've picked up time too. The free run shoes have less cushioning (particularly in the heel), to try and get you to approximate a mid-foot strike instead of a heel strike. A heel strike isn't bad in and of itself, but generally heel strike will lead to a straight leg landing which causes issues over time.
