If you have a laptop with a dvd player, then you wouldn't need to buy anything. If you feel comfortable traveling with it.
I used mine on our 11 hour drive last year. I also used it last spring on an airplane ride, about 4 hours. I think my battery was good for about 3 1/2 hours, but I'm sure they are all different. It also fit in the basket under the stroller through the airport.
A few years ago, we had a cheaper one from walmart and it froze up during play in the first day of use... they don't sell the brand there anymore, from the looks of it.. Later, I ended up getting myself a used laptop in Crys. lake and it worked out for us to use it as a dvd player as well.
I hope you find something that works for you. It definitely helped on the plane. We also took our own headphones, so we didn't bother others with it playing.
M