Hi Thommas, the mix I use is composed of several different ingredients, most of which you can find anywhere there is a hardware store or lumber yard.
You may have to search wider afield to get the peat moss, as that's more of a greenhouse or nursery item.
Mail order suppliers are a good place to try on the internet, especially in a major center like London where you might not have those types of stores close by.
Hope this helps, and good luck with your projects!