Want to be really cheap & you a handy person?
Go to you local glass shop & get them to cut 2 pieces of glass to fit exactly inside the the tank rim, but leaving room for your wires, filters, etc.. You would want a bigger piece in the back of the tank (maybe 2/3 of the depth), and a smaller piece in front. Glue some knobs/handles onto the glass with silicone for ease of handling. Place glass on top of tank. You now have a DIY glass cover! Use handles to remove front glass piece for feeding, & both pieces for cleaning. Some people silicone on hinges so you can just flip the front piece open.
If this is too much work, go buy a premade cover, but this will cost you $30 or more.
Now you can put any kind of light on top of tank. Go to your local home center & pick up a self contained undercabinet fluorescent light of the appropriate length (or whatever you like that will fit, but avoid incandescent light - too much heat & not enough light). Mount the light on a board. Make legs for the board so that the whole thing will sit on top of your glass cover with the light a couple of inches above the glass.
Alternative: hang the light from the ceiling, or overhead shelf with a long chain so the light is at the appropriate height.
To be really pretty: make yourself an enclosed hood to sit on top of the tank & mount your light inside. (Look through the DIY section for ideas.)