I get mine from Aardvark cleaning supplies in Warrington. If you are not local then search for a local supplier or alternatively search eBay or the interwebs. I pay £6.50 ish per 5 litre, if you are getting it delivered expect to pay more.
I personally always use it on conny roofs. It makes the job far easier and gives a nice shine. That is just my opinion and some people are happy using just tap or pure water.