Where can I get a replacement cooling fan

[Tumble]

For my HD4870? the stock one that's on it is starting to make a lot of noise, and I'm not comfortable having to get under the desk and tap it to make it stop. I think there's a bit of dust in the fan bushings, but I can't see any way short of destroying it to get it apart....

[LuckyNV]

Generally you can't take apart your cooler without voiding the warranty, but there are screws on the back, this removes the whole cooler, then there are more screws that holds the plastic shroud on, removing that will allow easy access to the fan, the bearing may be visible under the sticker.
In the past I've done the above and applied a tiny drop of oil into the bearing.

If its not under warranty any more then you could just buy a new heatsink and fan.

If the fan is faulty, then RMA, attempt to fix it yourself or install a new one, the latter can be tricky depending on your experience.

[Tumble]

The fan itself is removable without getting the heatsink off - held on with three small Philips screws. I'll have to hav another crack at it and see if I can get the fan apart

[LuckyNV]

Check the back, there should be a sticker you can remove, under that might be access to the bearing.

failing that, I've seen people ghetto mount 60mm/80mm fans above the fan hole so it blows air down into it and thus through the heatsink.