You can buy replacement parts from the manufacturer. Please call 800-932-3343 for more information.
No, you can't buy these individually.
The bulbs are 7 Watt incandescent low-voltage bayonet bulbs. (Apparently, 11 Watt incandescent low-voltage bayonet bulbs that fit the same sockets are also available.) If your purpose for using an LED light bulb is to save energy, don't do it. These lights are designed to use incandescent bulbs. The extra heat from the incandescent bulbs helps keep the light fixtures dry.