Hey. There's no stores anywhere around that carry promix or an LM-3 variety.
I find this to be a great soil and then I supplement 1 tablespoon of lime per gallon of soil. 3/4 tablespoon of bone meal, 1/2 tablespoon kelp meal and 1 tablespoon garden tone. I also use mykos in them. (spelling)
I add oceans forest too but you don't need it if you are feeding liquid nutrients through veg and bloom. Just makes them grow faster.
If you are able to find miracle grow I would think someone would have a bale of promix or like product.
Hopefully you get some better soil figured out as miracle grow will toast them in the middle of flower and you'll end up with stretching in the bud and it will become airy. You'll see stem through the colas. It'll look okay if it's your first time growing and you'll be happy you made a harvest but when you start to dial in you'll see how toxic miracle grow is to Cannabis. Reason being are those little blueish green balls in the soil are time release balls of nitrogen that wear down during every watering and then release their burst of nitrogen randomly. And it just so happens about 6-10 weeks after you start watering it that's when the bulk of it breaks down which would be contaminating your soil right through the beginning or critical weeks of flower resulting in an abundance of nitrogen being forced into the root system. You can't flush this nitrogen out either as the more water you try to use to flush it the more it releases nitrogen.
I hope this makes sense and you can find some better soil brother.
You'll definitely be able to grow smoke with miracle grow but if you stick with growing you'll look back and be like oh my god I should have stopped that sooner. Or not at all ;-)
I still think they will be perfectly fine though dude