Check out rangerovers.net. Here's what they say there (however dated it may be):
Windshield Washer/Wiper System:
I had to get a new windshield washer pump for my 89 Range Rover early on. More recently I have found this part for $39.95 at Atlantic British, and a replacement for $15 at ExpressAutoparts.com. (Select brand "Land Rover", and look in the "wiper" category).