You can use a Catholic Rosary, as is, for your Rosary magic. If you have a Catholic Rosary you want to convert into A Witch's Rosary, all you have to do is remove the strand of beads which protrudes from the main circle and which holds the crucifix at its end.
For a Witch's Rosary, all we need is the round part.
Depending on how your Rosary is constructed, you may be able to do it yourself with some needle nosed pliers. You can also take it to a jeweler to have it done for you.
Of course, there's never anything wrong with using a Catholic Rosary as is if you desire.