The lava lamp bulb is actually the traditional 40-watt frosted light bulb that you can find anywhere. Therefore, it is unnecessary to order pricey light bulbs from lava lamp manufacturers to replace your lava lamp bulb.
However, if you have one of the larger lava lamps that require 100-watt bulbs (which may be harder to find) or you just feel better buying replacement bulbs made for the lava lamp you can get them from Planet Lava.