I grow Rosemary all summer long in pots. I have found that these pots typically do not survive the winter indoors well. But I have a trick that I have used for years. I take cuttings from my pots and strip some of the woody stem away. I then place these cuttings in a jar of water and place the jar in a southern window over the winter. I then clip the Rosemary during the winter to use in my cooking. Sometimes it even will blossom with purple flowers.
The cuttings will root over the winter months in just plain water. Around March I transplant the cuttings into pots and then in about 4 weeks, I plant these pots in the large pots outdoors. I never have to buy Rosemary using this technique.