Get yourself down to an electrical wholesaler... every town has at least one, and ask for a "florry fitting" size depends on your tank, but common sizes are 2,3,4,6,8 feet... although others are available.
These "fluorescent fittings" are what you have in the kitchen etc... and are a fraction of LFS prices, I use a 2 foot one over my shrimp tanks... £8 !!!!
Although if you want to grow plants, you may like to swap the lamp for either a "daylight" or aquarium spec one.