Just head for the village in Wutai. Yuffie headed there after stealing your materia. Don't worry, things work out
Most of the monsters in that area drop those items - Bolt Plumes and whatnot - that let you use a single magic spell.

I always get Yuffie just after leaving the Mythril Mine. She has some fun scenes early in the game, plus it helps to get her as soon as possible if you (SPOILER)want her to date Cloud.