Newegg... Hyper 212+ Very cheap and very effective and you don't need to buy anything but the cooler as it even comes with new thermal paste.
As for the stock heatsink holding those clocks? I have heard of 4.0Ghz on the stock cooler. I just wouldn't go to much above 70C in temperatures. Watch the temps and don't fry it and you should be fine.
I'm still going to say get the Hyper 212+, excellent performance for a very budget price.