Need light replacement!!!

#1
My fluorescent light including the fixture for my 20g broke a week ago. It’s an All-Glass Aquarium, Inc. 24” Fluorescent Aquarium Reflector. It uses an 18” light bulb. I have a new bulb but the whole fixture is broken. My dad says he just wants to replace the ballast (little white box). How much do they cost? Who sells them? Or do I have to get a whole new fixture? Thanks for any help.