You can find keyfobs on Amazon or on eBay for about $80, but they'll need to be programmed.
The dealer won't just program any key; they'll program their own. It's not cheap.
You can take the $80 keyfob and program it yourself with either the iiD Tool from GAP Diagnostic or (purportedly) the iLand tool from Atlantic British. Those options will approach the dealer cost, but you'll keep either of those tools which prove extremely handy for the Rover owner.