Got my 4.5-10U 3DS XL from Walmart today. They have glass cases so you can see most of the serial numbers pretty clearly. They had 5 total, 3 blue, 2 red, and just one blue beginning with 1271. Was nice because they're giving away $50 gift cards with each purchase right now as well. Although I can't use it on any 3DS games because they'll force a mandatory update and I'm waiting on my Gateway cart in the mail.
Anyway, if most of your Walmarts are sold out after the holidays like mine were, try Target or K-mart, which are large chains but lesser used for stuff like electronics.