Indoor Herb Garden in the Winter

I would like to start an indoor herb garden but it is already December. Where can I buy plants? I'm worried that if I start from seed it will be spring before I can harvest anything...

I agree with you that this time of year live plants are best, but it is too cold to ship them. If you live in the Toronto, Ontario, Canada area- then all you have to do is drive to our greenhouse or take the GO-bus -it will stop at our door. Staff in the greenhouse will help you find the plants you want quickly.
However, if you are too far away then you may have to phone various local garden centres to find out if they are still open and if they stock some herbs. The yellow pages should list garden centres if you are in Canada. Otherwise you are stuck with the somewhat limp cut herbs of the produce section of your local supermarket!
If your house is warm, you could try some seeds that are moved to a south window as soon as they germinate. Basil and other annuals should be fast enough to give you a harvest, but with perennials you will probably get your first harvest in the spring at the earliest- in other words you might as well wait for the plant shipping season!