You appear to be getting 90+% isopropyl alcohol confused with denatured alcohol, which is ethyl alcohol with additives to make it undrinkable (like methyl alcohol and a very bitter chemical). Methyl is the stuff that blinds and kills you.
Isopropyl alcohol can be found in most drug stores. You may have to ask for it in the 90% concentration.
Denatured alcohol can be purchased in most hardware and paint stores.
If you are legal drinking age and the liquor stores are open on Sunday in your location, you can buy ethyl alcohol under the brand name of "Everclear."
As I do not use MM Acryls, I can't tell you what other thinners work well with that brand.
Artist supply stores sell various acrylic retarders.