I'm actually a Dept. Manager at the Woodburn, Oregon WalMart, and Pokemon cards fall under my jurisdiction, so I could actually be helpful for once!
First off, Crazar is absolutely correct. Just like with soda and most of the candy bars, we get trading cards through an independent vendor (ours is Vintage Sports Cards, inc.). Tony, the vendor for that company, told me that he only comes in roughly once every week, so that might account for some of the store's stocking issues. I should note that we've only had this vendor for a couple of months, so they're still trying to work us into their schedule.
Now, even before I was a manager, I always noticed that we would consistently be out of the most recent set of PKMN. So I suggested to the vendor that they give us double the usual quantity of the most recent set, and more than usual of that weird promo pack thing that retails for around $5 (the one that has the Mothim promo, among others). Hopefully, he can get us some more.
So, yeah, come to the Woodburn Wal-mart in a while. Maybe he listened to me. : )