is it OK for a non-catholic witch (im devoted to mary) to pray a rosary or use chaplets? if not, is there an alternative?
Many non-Catholics use prayer beads, so that would not be an issue as long as you fully and completely agree with the prayers that are to be said.
What strikes me is that you mention a devotion to Mary. The Virgin Mary does compel us to say the Rosary to bring us closer to her Son. If you are only devoted to Mary and not to Christ, I feel that she would find it highly disrespectful to do so. I will honestly take this a step further and say anything close to worship of the Blessed Virgin Mary would be disrespectful even if you are devoted to Christ as well, because I feel she would be quite upset to be put on a higher, or the same, level as Jesus.
Prayer beads are not exclusive to the Catholic faith. In fact, many religions around the world use them. And you are more than able to design your own set and own prayers to worship or praise whatever deity you like.