First of all, if you live in a northern climate, try to find cold-hardy garlic. More often than not, this will be the Allium sativum ophioscorodon, or hard-neck garlic. It may look a little bit like a flower but it is not as it produces small bulbils which will grow true to the variety when planted.
Garlic seed is actually one of the cloves from a head of garlic.
Garlic growers usually save the largest and best formed heads of garlic to use as seed for re-planting their crops. This ophio' or hard-neck garlic will grow and produce really well in the colder climates. However, it is important to get seed which has not been treated to prevent sprouting. Since garlic is difficult to store and sprouting is one of the problems most often encountered in storage, large growers and brokers will frequently treat the garlic to prevent that. One of the best places to buy high quality, untreated seed is at the Hudson Valley Garlic Festival held in Saugerties, NY the last weekend in September of each year.