I don't know of any after-market ones that are likely to make a big difference, why not just point a fan at it?
Ek do a really beautiful northbridge block if your budget will stretch to it. Sensible choice since you'll have to replace the sb and mosfet coolers as well, since they're all attached together
Yeah, by 'stretch your budget' I meant a radiator and pump as well.
Do you have a photo? Looking at mind it's beautifully set up for the intel stock cpu fan but not so good for tower heatsinks. You need a fan blowing horizontally through the fins (horizontal meaning parallel to board), one blowing down from above will do little
If blowing a fan across the heatsink makes no difference then my first thought would be that the heatsink isn't making good contact with the northbridge. Either reseat the northbridge cooler with something non-conductive like MX-2 (don't use AS5, it could conduct electricity in a bad way, but it's unlikely) or RMA the board.