Many people here build their own soil mix. The most strongly advocated here seams to be 5:1:1 mix. or its cactus brother the gritty mix. If these are of interest just use the search bar on top of this page. At the very least you'll need a good quality cactus mix to which you want to add extra perlite and or gritty material such as crushed granite. You want a fast draining mix. You might also consider using a unglazed terracotta pot. If outside for the summer, you'll want to protect it from the rain to better control the watering.