Fixtures at the LFS are expensive, but easy; they're made to fit the tank, and you don't have to do any work. If you're a do-it-yourselfer and you want to put some elbow grease into it, you can save a lot of money.
You can get shop lights at Home Depot - they're 1, 2 or possibly more tube 48" strip lights - and you can also purchase bulbs there. It's not the most attractive setup unless you can build a bit of a hood around the light fixture, but it will work. You could also purchase ballasts, end caps, wood, wire etc. at Home Depot and build your own hood - there are lots of plans available online, just Google 'DIY aquarium hood' or something similar.
If you're planning to keep high-light plants you may want to look into compact fluorescents - they give off more light than regular tubes and work really well for plants (but they're not cheap).