If you have the time and patience, a good BS detector, and live in a populated enough area, Craigslist. Fayetteville's CL looks active enough to me.
If you have the time and patience, and a good BS detector, Ebay.
If you have the time and patience, surf slickdeals.com forums.
If you have $500+ dollars, surf slickdeals or newegg and grab the best available today.
If ALL it needs to do is run quickbooks, any laptop can do that. Given the infinite models/specs of laptops, it can be hard to say what any given one is worth. Since, any pc can run just quickbooks, going "used" will get you what you want for bottom dollar. What I'd do if I found a model I was interested in that's a few years old, is search google and ebay for what their going rate, and if they have any known serious flaws, " 'model' problem(s)/issues"
And if it doesn't need to be portable, don't dismiss desktops.