In my experience, with popular titles, I've never been able to get it from GameStop within a few days of release unless I'm there immediately when the store opens/when the shipment comes in. Then, I go to the closest store that sells games, and they'll have several in stock.
When this has happened, GS has used it as a way to try to push preorders on me, saying they often have sell-outs, but if they've sold out and all the other stores haven't, then it's clearly manufactured.
That said, in this particular case, it seems NSMBWii is sold out at other places according to other posters. Since this is the case, it could be a genuine supply-could-not-reach-demand issue, rather than a manufactured GS one. I'd still check somewhere else, first, because I don't trust GS over things like this anymore.