Odyssea makes a good dual bulb t5ho fixture for planted tanks, I think it runs a little under $40? I've heard a lot of good things about it, especially at the price it's at. It's hard to find anything much lower priced than that, that's good for planted tanks atleast.
I'm not much help on the DIY part of things other than if you do go that route, you'll probably spend about 1/2 what you would on the odyssea on bulbs alone. Reflectors are where you'll want to focus most of your DIY efforts as well.
other than that all I can suggest is to check out your local craigslist like someone has already mentioned.
I haven't ever found anyone selling aquarium lighting seperate, but that's in my area and we don't have a lot of fish hobbyists around here.