There is a 0% chance that Wal-Mart and Target cards are fake. Most fake cards are sold at hobby stores (that may or may not know better), thrift stores, online, at flea markets, etc. I remember seeing fake cards sold in ziplock bags at a surf shop when I went to the beach back when Pokemon first came out.
Pokemon TCG increased by 400% in sales, so counterfeits will probably be coming back. The best way I used to detect fake Magic packs was the "tear" method. Take one of the commons from the packs and rip it in half. If there is no blue paper in the middle, the packs ARE fake. I'm pretty sure Pokemon cards use this type of paper too, you could try opening a common you know is real to find out.
If you buy non-foil singles, put one end against your thumb and the other end against your index finger and bend it til the ends meet. 99.999% of fake cards will be damaged by this; real ones will bend back into shape. Don't try this on foils, as I don't think it would bend back properly.