As jampot says, you can get CTs from any homebrew shop or online supplier, you can also get then from Wilkinson's. They are a convenient measured dose of sodium metabisulphite, which will get rid of any free chlorine or chloramines which your water company may have put into your supply.
The trouble is, you will have to buy a pot of 100 for around £2 and in brewing you will only be using half a tablet to treat 5 gallons of water, so that pot should last you a while.
Unless you use a forum like this one you won't even hear about water treatment, I seriously doubt that you would find any brew kit which mentions anything about water treatment in the instructions, so it's not strictly necessary.
If you're happy to drink your tap-water âneatâ then you probably don't need to pre-treat, it's just an added refinement.
If you use bottled water you don't need to treat for chlorine, but that doesn't necessarily mean that bottled water is better, it could be deficient in other things which might be beneficial.
And if you didn't want to take a peek at your first brew from time to time you wouldn't be normal, just try not to make too much of a habit of it.