Not CMS but CRS (carbonate reducing solution) can be found in many homebrew stores (
here for example) but to use it you need to know what the alkalinity or bicarbonate of your tap water is. I don't think you'll get this info from NI Water but you can test this with a Salifert KH kit.
I wouldn't randomly use gypsum or CC either without knowing at least the calcium content of the water (and preferably also the sulphate and chloride).
The Campden tablet is one that I would definitely use, roughly half a tablet per 35L will remove all the chlorine. There's no need to boil the tap water before you use it.
Personally I think learning to use your tap water is much more preferable to using bottled or filtered water, though I understand that that's not always practical.