To me, a "mix" is something to which you do not add...
Thus, I have two orchid "mixes" : Prime Agra for s/h and Aussie Gold (medium)
Other than that, I use the individual components: NZSpag, Perilite, Charcoal, Bark....
Mostly from repot me.com
I grow mostly catts but also phals, oncs, two phrags, one paph, a few vandas and other strays.
I agree stocking the components and making your own mix is best
I do this not to save money, but to make up what I think the plant needs.