If you haven't ridden a bicycle in a while, buy from a serious bike shop. It may cost more, but it ensures that the bike will fit you and be safely constructed. Plus, most bikes sold from shops come with a few months of free service.
Department store bikes offer lower prices, but don't have much in the way of quality or reliability. Only purchase these bicycles for uses where they will be replaced often, and not ridden for long distances.
If you've been riding for a while, and know what you're looking for in a bike, Internet outlets and auction sites can offer you great deals. Just be sure to protect yourself by doing some background research on the seller before buying.