you get shell from barrier materia.

and it is possible to win the game at your level, though it may be a little difficult. (you could cheat and clone countless elixirs, megaelixirs, and hero drinks with w-item, provided you got it. )

feed elixirs to the magic jars, in the northern cave. (you can level up ALOT there. ) i got to my 70s in a day there

but it still is possible to beat the game there.

add on: also, if you level up barrier materia enough, you get wall, which has the effects of barrier and shell. (which are called barrier and m-barrier.)

or you could manipulate beach plugs at the beach near costa del sol...and get big guard. same effect as wall.

hope that helps.