You could do something like this for pretty cheap.
For a bit more you could go with something like this.
Not sure if either will get you the maximum amount of light you can have with out needing Co2, but they will grow plants. Alternatively you might just want to spend some money up front and get a proper light like this...
But that isn't the best light out there either. These are just some ideas...on a 48 inch tank you can spend anywhere from $50 to $500 on a light setup. The choice is yours. I'm running two of the Home Depot Shoplights. They work very well, but they are shoplights and perhaps not as visually appealing as other lights. Sometimes I wish I had just gotten a fancy fixture, but only because the overall setup might look a little cleaner.