The light bulb that's usually in a lava lamp is a 40 watt appliance light bulb. You can find them about anywhere. Here are some tips on removing the light bulb since it sometimes doesn't want to come out:

1. Unplug your lava lamp since you might not know if it's on or off if the bulb is dead.
   
2. Use dishwashing gloves. The Mr. Clean ones are extra grippy and work well.
3. Keep twisting it back and forth. Eventually, it should work itself out more and more.
4. Last resort may be to break the bulb and use a potato to grab it out, but you shouldn't need to do this and I don't suggest doing this.