A galvanized chassis insures longevity regardless of geographic location. The difference in cost is completely negligible. The addition in $ value to a future prospective buyer for the piece of mind gained far far exceeds the the present day cost differential.
IMO if you you are going to extreme of changing your chassis it would be fool hardy to not use one thats been dipped.
To answer your original question, a "replacement chassis" can be ordered from any Land Rover dealer and this will come from Solihull. It will come painted black. Is it more expensive than an aftermarket galvanized one ? Absofrigginglutely.
FWIW- we sandblast and dip "replacement" chassis that we get through Land Rover, because that improves on the original design, something we try to do whenever possible.