Walmart offers Mario Kart 7 for Nintendo 3DS for $29.96. (Amazon charges the same for Prime members.) Choose Site-to-Store pickupto dodge the $4.97 shipping fee. That's the lowest price we could find by $10, although we saw it for $12 less last month.
Orders placed by December 19 are guaranteed for Christmas availability.
Update: It's now available for in-store pickup only.