For taking portable DVD players in the field, I suggest looking at
stores or websites like bestbuy.com and circuitcity.com or compusa.com,
etc. I find that electronics type stores have better quality than other
places that sell them like Walmart, Kmart, etc.

I have seen them on clearance for as low as $50, but you should plan to
spend $100-$300. When shopping, I suggest going a step or two above the
lowest priced items.

Things to look for: at least 5" screen (any smaller is too hard to see)
and good speakers, although speakers are not as important because most
of them have a headphone jack. They should all also have battery packs
and you can get accessories to plug them into a car lighter jack.

I am still waiting for a truly durable DVD player to come out that you
can throw in your pack like a book. Keep in mind that they are
electronic devices that should be handled with care in the field, like
you would with a camera, video camera or spotting scope. They work very
well as a resource to take with you in the car while traveling.

Hope this helps,

Joe LaFleur
Better Birdwatching