These are all pretty good cars, however which is the best car to look at in this
category. Nissan: Altima? Maxima? Toyota: Camry? Avalon? and where should I
search online for good quality used cars. (Besides autotraders.com

I like cars.com. Good luck

You answered your own question and I agree they are all good, as for choice, your lifestyle for size and handling however may not be mine, it really is a question only you can answer for yourself,,, but if you must know the Maxima fits my lifestyle, and on the other hand the Altima would be perfect for my wife.

As for a search online besides auto trader your hitting a negative with me, let me explain why.

Over 90% off reputable dealers list with auto trader, they are the leaders, everything else you will get on the Internet, will be from third party Internet providers or some listing on ebay by scammers