Where can i buy a certain light bulb

so i have a lightbulb powered lava lamp that needs a lightbulb. the port where it screws in is about half a inch in diameter. it also needs more fluid, so can i use water or what do i need

It is probably a candelabra base, which is more common than an intermediate http://ext.homedepot.com/shopping-tools/…
and you should be able to find them where decorative bulbs are sold.
It shouldn't be losing fluid and that may mean the cap is loose. Distilled water might work
http://www.mikegigi.com/lavasubs.htm