As far as I know, if it is possible to buy the cooler part alone, it would only be a main dealer part. For that reason it would make sense to buy the whole thing, which would probably work out cheaper anyway. If there's corrosion at the bottom, it surely follows that the top isn't far behind. A complete radiator can be bought for £85 upwards.
phil do they come complete with the ATF radiator aswell? do you have a link?