Cindy, if you take the bulb out and take it to any pet shop they should have the tube. I had one that was on a 5gal tank and it was like a 12" 8watt bulb or something and they had several color temperatures to choose from at Pet Supplies Plus which is where I went. I bought a 12,000K and it had nice color. I think they make a 20,000K if you like more blue.
You need the numbers off the bulb to know the size and wattage. Most of the time it has a "T" number then a wattage like T4-8w.
I think I paid $15.
HTH
-- Kevin