Can someone tell me where to get it?

at the shinra mansion get the destruct materia and level it up to get debarrier.

Sephi throws the Destruct materia at you on your first visit to the Shinra Mansion. (not the flashback, the first ACTUAL visit)

You get DeBarrier as a initial spell without any AP.

yeah you can also by it in some shops like in cosmo canyon i think but some materia shops have it as well

I am just trying to think, but I think, it was in the shop of the Rocket Town (but can't remember very well right now). ;)