As far as I know you cannot use any ZR window regulator on ZS as they are different. However you can use any Rover 45 or Rover 400 '95-'99 as they are the same part.
Firstly
There are a few people that frequent certain Facebook groups that sell spares. Try DMGRS.co.uk as they may know of spares or stock availability? I may have a spare but it’s currently under cover and not accessible, not helpful I know!