Nico Nico Nii!~
I just buy all my booster packs, tins, theme decks and individual cards online. Mostly from Toy Wiz, Troll & Toad, PokeOrder, & BB Toys. If I can't find an individual card I want I'll go with my final choice ebay.
For Pokemon cards at Wal-Mart and Target, try by the registers where the Magic: The Gathering Cards are. This is where I've found them 99% of the time in these two stores. Though you usually won't find any packs older than a year old at Target.