Wanting to do Subs Supersoil, any comperable alternatives to roots 707 for base soil?

Im wanting to do a batch of supersoil but roots 707 is not carried normally at my grow shop. What are some alternatives for a base soil? Could pro mix be used? Would additional amendments be needed for the pro mix to be used? Any and all advice is appreciated! Thanks in advance.

its roots original and I hear happy frog is good I use roots original my self

I use the Pro Mix HP. I start with 1- 3.8cu ft. bag add 2- 50L bags of Canna CoCo and a 30Lb. bag of Wiggle Worm earth worm castings. Once all your other ingredients are mixed, it should almost fill 2- 45 gallon garbage cans.

roots original
roots greenlite

i mix them 1:1 for my mix and add castings