I'd recommend starting with cars.com and autotrader (along with doing a little pricing research on Kelly BB and Edmunds). At a minimum, you get a good feel for what you should be paying for the type (and age/miles) of vehicle in your particular area. Dealers and private sellers both post on those sites, so you're basically killing 2 birds with one stone. I'd look at Carmax (start with on-line), and if you're feeling more bold in the internet age, you could also check Carvana. Another benefit of starting with these sites is you're typically able to get a free look at the car's carfax/autocheck report.
With so many people leasing these days, there tends to be a pretty significant number of 2-3 year old vehicles in that 30k mile range that just came off lease, and businesses dedicated to selling that type of vehicle. While these types of dealers will have their own sites, they also usually leverage cars.com and autotrader to market them.