LOL. You're in the same spot I was in last year. I was getting the Wii for my oldest son and I couldn't find one anywhere. WalMart kept telling me there were more coming in but they couldn't tell me when. I would just have to keep coming back.
I did sit outside a Target for several hours in the cold. (something I said I would NEVER do) When they finally handed out the numbers, they stopped 2 short of me.
Everyone in line had a good time though.
I ended up giving my son an IOU till after Christmas when they came in. He was so excited. (He was 21!) The anticipation was fun to watch.
I got one in January and wrapped it up and called my son and told him I had something he wanted.... he was over to the house pretty darn fast. It was almost worth the wait to see how excited he was.
I hope you can find one, though. If your niece and nephew are young, they may not be as willing to wait!