I highly recommend checking out a short list of models in advance, choosing one to target, then watching everywhere for deals on that specific model. Once you know which model and year you're targeting, you could just stay on the lookout for that everywhere...Carmax, autotrader, carvana, etc.
Since you asked about Carmax...not the absolute cheapest option but I've bought two from them. Both in great condition and ran forever. I liked the overall experience and the selection (one car was delivered from KC to Nashville). It's also a great place to just go SEE the cars, check out how the interior feels, and test drive several to determine which one you want to focus on.