gypsum boards for ceiling use;

when i browse for some model houses, they often indicate they use 10-13mm gypsum boards for interior ceilings and magnesium boards for the outside.

besides the usual depots, where else can i get gypsum boards?

mga 10 pcs lang to repair a section of the house (using it to try its better heat insulation and being less combustible) and its cheaper daw vs. marine plywood and hardiflex.

10 pcs parang mahal kung padeliver na may bayad. and i think their fragile enough that i cant load them in my pickup.

and is it really a direct alternative to plywood for ceiling use, keeping in mind it will be on a DIY carpenter skill only (so it will be screwed or fastened to the wood in place of plywood without additional stuff??)

thanks