buy some of that,

and this
https://www.amazon.com/gp/product/B0762X8QQ1/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
the latter is a substrate... it's practically like fertilizer, very valuable. mix them together in your own ratio, whatever you find affordable or to your liking. If it's for bridgesii or something like that I use more regular soil and less substrate. for peyote, I use more substrate, but

cactus soil mix (my favorite brand is Black Gold) on the top for a seedling. they'll grow into the 1/3 pumice 1/3 lava 1/3 akadama rocks soon enough

cheers bud good luck