You can get large bags of Perlite from any greenhouse/ nursery supplier.
I get my Fafard Pro mix and Perlite from
http://www.carlinsales.com/
BTW ....... the correct name for the Miracle grow and others is Potting
Mix.
You do not want potting soil it is just about useless for fast growing tropical giants .
The only thing worse is
totally useless Top Soil.
Then you end up adding compost,bark,vermiculite etc to beef it up .
MG or other brands of potting MIX are made only from these additives to begin with.
If you want to lighten up your Potting Mix you can add some vermiculite or perlite. Be careful which you choose .
Verm holds water but Perlite does not.
These 2 additives will not change the nutritional profile of your chosen Potting Mix.
Adding peat, compost or composted manure will change the profile.
heeeeeeeeeee, fell off my soap-box ........... I'm outta here
S