yep gamestop is your best bet. or gamecrazy (most are closing though). there's a ton of gamestop stores so if one doesn't have what you want, try another. they're generally not hard to find used, especially now the new ds is coming out.
I agree, Gamestop is the best place to get a DS lite for under retail. Sometimes they even have trade in deals going on where if you trade in certain items you get an x% of extra trade in credit. The return policy isn't actually 7 days, it's 30 days if you change your mind and/or if you encounter any hardware deficiencies. I used to work at a Gamestop, and if the SA is nice, they'll let you play around with the unit for a while in the store before you purchase it.