Where do you buy rosary?

You can buy rosaries at religious stores, Catholic stores, online, or at thrift stores but I would recommend going to a Catholic church and ask for one from the priest. The priest can even bless it for you. Sometimes there are rosaries at the front entry of Catholic Churches that are for anyone who want to pray the rosary. You can also make one out of cord. Do a web search for cord rosary and you will find out how to make one.