It was originally in a mix I bought from someone on eBay who hand made all their soils, but I wasn't too keen on it. Now it's in a 50/50 mix of perlite and this soil: https://www.amazon.co.uk/Levington-Desert-Cactus-Bonsai-Compost/dp/B00IPT9F36/ref=sr_1_1?ie=UTF8&qid=1512051445&sr=8-1&keywords=levington+cactus+compost
I use this soil with about 40% perlite and 20% potting grit, which I find harder to find online. The soil is pretty sandy, so you really need the perlite and grit to stop it getting claggy. I don't know where you live, but garden centres should stock all three of these- for much cheaper than you can buy them online. I can send you some recommendations if you're in London?