Check out the following stores...
Circuit City,
Best Buy, Wal-Mart, Target and SAMS
I got mine a Circuit City. I did not go cheap and bought a good one, but with the price match policy I ended up saving well over 100.00 bucks. I bought it originally at 699.00. It has a 9 inch screen. Had a rebate for 75. So I got it for 624. I saw it on sale a month later there and it was 524. They matched the 100 bucks plus added 10%.
Dont go real cheap on them.
Some of them crap out easily. Do some research on them.
Mine is a Panasonic and is fantastic.
Next choices would be Sony, Toshiba, Audiovox, Mintek, GtG.
If your have two kids and they will be sharing, you wont want to go any less than a 7 inch screen. The GTG has 2 monitors on them. Both are 7 inches I believe. Saw that one at SAMS. I tried to research them but there wasnt anything out about them.
Good luck...
PS...They are worth their weight in GOLD!!!
17.5 hour car trip and 5yr old son didnt complain once.