The wear is usually in the shaft, hence why the seals don't work. You can pick good dry boxes up from eBay or buy the ?200 recon ones from your local independant. Some recon boxes are better than others though - the cheapest ones don't last very long if you are REALLY using your Land Rover!
If it's a minor weep rather than a dripping leak sometimes seal conditioner can sort it out (for a short while. . .)