eBay! or maybe the dealer, ive had a quick look at Rimmers and can't see any there but I only looked in the gearbox and interior trim sections so there might be one under another section. They cost quite a bit though, I know that much.
They do come up on egay from time to time, and they tend to be bloody expensive. Not sure if LR ever did a wooden gear knob but replacements are definately available.
Assuming its an auto box, its only a single allen screw that holds the knob on.