Not sure about all places and all stores, but Toys R Us said they only put them out on Sundays. Even if they get them mid week, they will wait until Sunday. They said to call on Sat night and they will let you know if they have any for the next day.
Thats the one i have had the best of luck, I think Ever store in the Toys-R-Us Chain does this practice. They are one of the larger ones that process orders froma central warehouse. If your looking at stores i would say this is you best bet.
Other large stores that use there own warehouse system seem to ship them out when ever they get them, so there is no set date on when they show up. Best bet is find a store where you know someone that works there and get the heads up ahead of time maybe even have them set one aside.