You didn't specify if you were getting an adult or baby so I will cover both. Turtles under 4" are not legal to sell because smaller kids supposedly will mouth them and get salmonella. See
http://www.fishpondinfo.com/turtles/babyintro.htmSo, if you're going to get a turtle, it will have to be over 4". That leaves out the smaller species. You also probably want a turtle that doesn't get over a foot long, isn't expensive, is easy to care for, is native to the US, and doesn't bite. So, your best bet is either a red-eared slider or a painted turtle. Both will require eventually a tank of at least 40 gallons if not twice that as well as at least a half hour of physical (adult) labor at least once a week to do it right.
From my web site regarding buying baby turtles specifically but also about that company you mentioned:
"I have been asked where you can buy baby turtles. Before now, I have refused to reply but many people just "must" have a baby turtle no matter how they get it which is sad. There are two places that I know about. I do not understand how they can operate legally as it is illegal to sell turtles under 4" in all of the USA. They say it is because the turtles are "adopted" but they charge money. I question the legalities and ethics. I have also heard bad stories about babies bought from the first place listed where the turtles were sick, listless, and/or dead. That, of course, may be the case with any mass-sold baby turtle operations and not unique to them. That site also stole a photo from my site that was sent to me in good faith from someone else who gave me alone permission to use it. I do NOT recommend that you order from these places but will provide the URL's."
You can look over my various turtle pages at
http://www.fishpondinfo.com/turtles/index.htmThis week, I'm getting a larger tank so my turtle (a red-bellied turtle) can get a bigger tank. I've spent probably $500 on him so far in less than two years. I hope someone in the family has the time and money to take care of a turtle because a six-year old is just going to be doing the enjoying part! My 2-year-old niece loves to watch my turtle eat. Good luck!